Two girls smiling

A girl is any female human from birth through childhood and adolescence to attainment of adulthood when she becomes a woman. The term may also be used to mean a young woman.^[1]

Etymology[link]

The English word girl first appeared during the Middle Ages between 1250 and 1300 CE and came from the Anglo-Saxon words gerle (also spelled girle or gurle).^[2] The Anglo-Saxon word gerela meaning dress or clothing item also seems to have been used as a metonym in some sense.^[1]

A girl from Mali

Girl has meant any young unmarried woman since about 1530. Its first noted meaning for sweetheart is 1648. The earliest known appearance of girl-friend is in 1892 and girl next door, meant as a teenaged female or young woman with a kind of wholesome appeal, dates only to 1961.^[3]

Usage for adults[link]

The word girl is sometimes used to refer to an adult female. This usage may be considered derogatory or disrespectful in professional or other formal contexts, just as the term boy can be considered disparaging when applied to an adult man. Hence, this usage is often deprecative.^[1] It can also be used deprecatively when used to discriminate against children ("you're just a girl").

In casual context, the word has positive uses, as evidenced by its use in titles of popular music. It has been used playfully for people acting in an energetic fashion (Canadian singer Nelly Furtado's "Promiscuous Girl") or as a way of unifying women of all ages on the basis of their once having been girls (American country singer Martina McBride's "This One's for the Girls"). These positive uses mean gender rather than age.

Demographics[link]

An adolescent girl from Mauritania

Slightly more boys are born than girls (in the US this ratio is about 105 boys born for every 100 girls), but girls are slightly less likely to die than boys, during childhood, so that the ratio for under 15 years of age is 104 boys for every 100 girls.^[4][5] Since the 18th century the human sex ratio has been observed as about 1,050 boys for every 1,000 girls born and sex selection on the part of parents further lessens the number of female births.

While patriarchal beliefs have led to female infanticide (the killing of female infants) since ancient times, it is a persistent practice in some regions of the world today. It is a significant problem in India and China in particular. The United Nations Population Fund (UNFPA) State of the World Population Report indicates that these practices, combined with neglect, have resulted in at least 60 million "missing" girls in Asia. Large numbers of adult women are absent from the population in some regions, a consequence of female infanticide during recent decades.

Census information shows the problem is worsening. In India overall, by 2011, there were little more than 9 girls younger than 6 years old for every 10 boys. The 2011 census showed that the ratio of girls to boys under the age of 6 years old has dropped even during the past decade, from 927 girls for every 1000 boys in 2001 to 918 girls for every 1000 boys in 2011. Maharashtra state's ratio is 883 girls, and Satara is even lower at 881. Hospitals in India are banned from giving out the gender of an unborn fetus to prevent sex-selection abortions, although evidence indicates that the information is often revealed.^[6] Steven Mosher, president of the Population Research Institute in Washington, D.C. has said: "Twenty-five million men in China currently can’t find brides because there is a shortage of women [...] young men emigrate overseas to find brides." The gender imbalance in these regions is also blamed for spurring growth in the commercial sex trade; the UN's 2005 report states that up to 800,000 people being trafficked across borders each year, and as many as 80 percent are women and girls.^[7]

Biology[link]

Girls develop female characteristics by inheriting two X chromosomes (XX), one from each parent.^[8]

About one in a thousand girls have a 47,XXX karyotype, and one in 2500 have a 45,X one.

Girls typically have a female reproductive system. Some intersex children with ambigous genitals, and biologically male transgender children, may also be classified or self-identify as girls.^[9]

Gender and environment[link]

A girl plays with paper dolls. Biological sex interacts with environment in ways not fully understood.

Biological sex interacts with environment in ways not fully understood.^[10] Identical twin girls separated at birth and reunited decades later have shown both startling similarities and differences.^[11] In 2005 Kim Wallen of Emory University noted, "I think the 'nature versus nurture' question is not meaningful, because it treats them as independent factors, whereas in fact everything is nature and nurture." Wallen said gender differences emerge very early and come about through an underlying preference males and females have for their chosen activities.

Girls' education[link]

School girls in Afganistan

Girls' equal access to education has been achieved in some countries, but there are significant disparities in the majority. There are gaps in access between different regions and countries and even within countries. Girls account for 60 per cent of children out of school in Arab countries and 66 per cent of non-attendees in South and West Asia; however, more girls than boys attend schools in many countries in Latin America, the Caribbean, North America and Western Europe.^[12] Research has measured the economic cost of this inequality to developing countries: Plan International’s analysis shows that a total of 65 low, middle income and transition countries fail to offer girls the same secondary school opportunities as boys, and in total, these countries are missing out on annual economic growth of an estimated $92 billion.^[12]

Although the International Covenant on Economic, Social and Cultural Rights has asserted "primary education shall be compulsory and available free to all" girls are slightly less likely to be enrolled as students in primary and secondary schools (70%:74% and 59%:65%). Worldwide efforts have been made to end this disparity (such as through the Millennium Development Goals) and the gap has closed since 1990.^[13]

Educational environment and expectations[link]

Kurdish girl

According to Kim Wallen, expectations will nonetheless play a role in how girls perform academically. For example, if females skilled in math are told a test is "gender neutral" they achieve high scores, but if they are told males outperformed females in the past, the females will do much worse. "What’s strange is," Wallen observed, "according to the research, all one apparently has to do is tell a woman who has a lifetime of socialization of being poor in math that a math test is gender neutral, and all effects of that socialization go away."^[14] Author Judith Harris has said that aside from their genetic contribution, the nurturing provided by parents likely has less long-term influence over their offspring than other environmental aspects such as the children's peer group.^[15]

In England, studies by the National Literacy Trust have shown girls score consistently higher than boys in all scholastic areas from the ages of 7 through 16, with the most striking differences noted in reading and writing skills.^[16] In the United States, historically, girls lagged on standardized tests. In 1996 the average score of 503 for US girls from all races on the SAT verbal test was 4 points lower than boys. In math, the average for girls was 492, which was 35 points lower than boys. "When girls take the exact same courses," commented Wayne Camara, a research scientist with the College Board, "that 35-point gap dissipates quite a bit." At the time Leslie R. Wolfe, president of the Center for Women Policy Studies said girls scored differently on the math tests because they tend to work the problems out while boys use "test-taking tricks" such as immediately checking the answers already given in multiple-choice questions. Wolfe said girls are steady and thorough while "boys play this test like a pin-ball machine." Wolfe also said although girls had lower SAT scores they consistently get higher grades than boys across all courses their first year in college.^[17] By 2006 girls were outscoring boys on the verbal portion of the United States' nation-wide SAT exam by 11 points.^[18] A 2005 University of Chicago study showed that a majority presence of girls in the classroom tends to enhance the academic performance of boys.^[19]

International initiatives for girls' rights[link]

Indian girls

Girl in Guatemala

An ahwazian Arab girl in traditional Arab dress.

The United Nations Convention on the Rights of the Child (1988) and Millennium Development Goals (2000) promoted better access to education for all girls and boys and to eliminate gender disparities at both primary and secondary level. Worldwide school enrolment and literacy rates for girls have improved continuously. 2005, global primary net enrolment rates were 85 per cent for girls, up from 78 per cent 15 years earlier; at the secondary level, girls’ enrolment increased 10 percentage points to 57 per cent over the same period.^[12]

A number of international non-governmental organizations (NGOs) have created programs focussing on addressing disparities in girls' access to such necessities as food, healthcare and education. CAMFED is one organization active in providing education to girls in sub-Saharan Africa. PLAN International's "Because I am a Girl" campaign is a high-profile example of such initiatives. PLAN's research has shown that educating girls can have a powerful ripple effect, boosting the economies of their towns and villages; providing girls with access to education has also been demonstrated to improve community understanding of health matters, reducing HIV rates, improving nutritional awareness, reducing birthrates and improving infant health. Research demonstrates that a girl who has received an education will:

• Earn up to 25 percent more and reinvest 90 percent in her family.
• Be three times less likely to become HIV-positive.
• Have fewer, healthier children who are 40 percent more likely to live past the age of five.^[20]

Plan International has also created a campaign to establish an International Day of the Girl. The goals of this initiative are to raise global awareness of the unique challenges facing girls, as well as the key role they have in addressing larger poverty and development challenges. A delegation of girls from Plan Canada introduced the idea to Rona Ambrose, Canada's Minister of Public Works and Government Services and Minister for Status of Women, at the 55th Session of the Commission on the Status of Women at United Nations Headquarters in February 2011. In March 2011, Canada's Parliament unanimously adopted a motion requesting that Canada take the lead at the United Nations in the initiative to proclaim September 22 the International Day of the Girl. If adopted by the General Assembly of the United Nations, an International Day of the Girl could be declared before the end of 2011.^[21]

Its most recent research has led PLAN International to identify a need to coordinate projects that address boys' roles in their communities, as well as finding ways of including boys in activities that reduce gender discrimination. Since political, religious and local community leaders are most often men, men and boys have great influence over any effort to improve girls' lives and achieve gender equality. PLAN International's 2011 Annual Report points out that men have more influence and may be able to convince communities to curb early marriage and female genital mutilation (FGM) more effectively than women. Egyptian religious leader Sheikh Saad, who has campaigned against the practice, is quoted in the report: “We have decided that our daughter will not go through this bad, inhumane experience [...] I am part of the change.”^[22]

Art and literature[link]

Peter Paul Rubens - Portrait of a Young Girl

The Time Of The Lilacs, by Sophie Gengembre Anderson.

Historically, art and literature in Western culture has portrayed girls as symbols of innocence, purity, virtue and hope. Egyptian murals included sympathetic portraits of young girls who were daughters of royalty. Sappho's poetry carries love poems addressed to girls.

In Europe, some early paintings featuring girls were Petrus Christus' Portrait of a Young Girl (about 1460), Juan de Flandes' Portrait of a Young Girl (about 1505), Frans Hals' Die Amme mit dem Kind in 1620, Diego Velázquez' Las Meninas in 1656, Jan Steen's The Feast of St. Nicolas (about 1660) and Johannes Vermeer's Girl with a Pearl Earring along with Girl Reading a Letter at an Open Window. Later paintings of girls include Albert Anker's portrait of a Girl with a Domino Tower and Camille Pissarro's 1883 Portrait of a Felix Daughter.

Mary Cassatt painted many famous Impressionist works that idealize the innocence of girls and the mother-daughter bond, for example her 1884 work Children on the Beach During the same era, Whistler's Harmony in Gray and Green: Miss Cicely Alexander and The White Girl depict girls in the same light.

The European children's literature canon includes many notable works with young female protagonists. Traditional fairy tales have preserved memorable stories about girls. Among these are Goldilocks and the Three Bears, Rapunzel, The Princess and the Pea and the Brothers Grimm's Little Red Riding Hood. Well-known children's books about girls include Alice in Wonderland, Heidi, The Wonderful Wizard of Oz, the Nancy Drew series, Little House on the Prairie, Madeline, Pippi Longstocking, A Wrinkle in Time, Dragonsong, and Little Women.

Beginning in the late Victorian era, more nuanced depictions of girl protagonists became popular. Hans Christian Andersen's The Little Match Girl, The Little Mermaid, and other tales featured themes that ventured into tragedy. Alice's Adventures in Wonderland by Lewis Carroll featured a widely noted female protagonist confronting eccentric characters and intellectual puzzles in surreal settings. Moreover, Carroll's controversial photographs of girls are often cited in histories of photographic art. Literature followed different cultural currents, sometimes romanticizing and idealizing girlhood, and at other times developing under the influence of the growing literary realism movement. Many Victorian novels begin with the childhood of their heroine, such as Jane Eyre, an orphan who suffers ill treatment from her guardians and then at a girls' boarding school. The character Natasha in War and Peace, on the other hand, is sentimentalized. By the 20th century, the portrayal of girls in fiction had for the most part abandoned idealized portrayals of girls. Popular literary novels include Harper Lee's To Kill a Mockingbird in which a young girl, Scout, is faced with the awareness of the forces of bigotry in her community. Vladimir Nabokov's controversial book Lolita (1955) is about a doomed relationship between a 12 year old girl and an adult scholar as they travel across the United States. Zazie dans le métro (Zazie in the Metro) (1959) by Raymond Queneau is a popular French novel that humorously celebrates the innocence and precocity of Zazie, who ventures off on her own to explore Paris, escaping from her uncle (a professional female impersonator) and her mother (who is preoccupied by a meeting with her lover). Zazie was also made into a popular movie in 1960 (Zazie dans le métro) by French director Louis Malle.

Books which have both boy and girl protagonists have tended to focus more on the boys, but important girl characters appear in Knight's Castle, The Lion, the Witch and the Wardrobe, The Book of Three and the Harry Potter series.

Recent novels with an adult audience have included reflections on girlhood experiences. Memoirs of a Geisha by Arthur Golden begins as the female main character and her sister are dropped off in the pleasure district after being separated from their family in 19th century Japan. "Snow Flower and the Secret Fan" by Lisa See traces the laotong (old sames) bond of friendship between a pair of childhood friends in modern Beijing, and the parallel friendship of their ancestors in 19th century Hunan, China.

Popular culture[link]

A teenage girl with a modern hairstyle in Harajuku, Tokyo.

There have been many American comic books and comic strips featuring a girl as the main character such as Little Lulu and Little Orphan Annie. In superhero comic books an early girl character was Etta Candy, one of Wonder Woman's sidekicks. In the Peanuts series (by Charles Schulz) girl characters include Peppermint Patty, Lucy van Pelt and Sally Brown.

In Japanese animated cartoons and comic books girls are often protagonists. Most of Hayao Miyazaki's animated films feature a young girl heroine, as in Majo no takkyūbin (Kiki's Delivery Service). There are many other girl protagonists in the Shōjo style of manga, which is targeted to girls as an audience. Among these are The Wallflower, Ceres, Celestial Legend, Tokyo Mew Mew and Full Moon o Sagashite. Meanwhile, some genres of Japanese cartoons may feature sexualized and objectified portrayals of girls.

The term girl is widely heard in the lyrics of popular music (such as with the song "About a Girl"), most often meaning a young adult or teenaged female.

History[link]

Princess Neferure as a girl, sitting on the lap of her tutor Senenmut. Girls and women in Ancient Egypt enjoyed a relatively high social status.

The status of girls throughout world history is closely related to the status of women in any culture. Where women enjoy a more equal status with men, girls benefit from greater attention to their needs.

Girls' education[link]

In Ancient Egypt, the princess Neferure grew up under the reign of her mother, the woman Pharaoh Hatshepsut, who had inherited the throne after the death of her husband Thutmose II. Women in Ancient Egypt had a relatively high status in society, and as the daughter of the pharaoh, Neferura was provided with the best education possible. Her tutors were the most trusted advisors of her mother. She grew up to take on an important role by taking on the duties of a queen while her mother was pharaoh.^[23] Despite the fact that women and men had a great deal of equality in Ancient Egypt, there were still important divisions in gender roles. Men worked as scribes for the government, for example, whereas women would often work at occupations tied to the home, such as farming, baking bread and brewing beer; however, a large number of women, particularly from the upper classes, worked in business and traded at markets, as perfumers, and some women also worked in temples. For this reason, girls' and boys' education differed. Boys could attend formal schools to learn how to read, write, and do math, while girls would be educated at home to learn the occupations of their mothers. Some women did become literate and were scholars, however, such as Hypatia.^[24][25]

The future Elizabeth I of England at age 13 years.

Girls' formal education has traditionally been considered far less important than that of boys. In Europe, exceptions were rare before the printing press and the Reformation made literacy more widespread. One notable exception to the general neglect of girls' literacy is Queen Elizabeth I. In her case, as a child she was in a precarious position as a possible heir to the throne, and her life was in fact endangered by the political scheming of other powerful members of the court. Following the execution of her mother, Anne Boleyn, Elizabeth was considered illegitimate. Her education was for the most part ignored by Henry VIII. Remarkably, Henry VIII's widow, Catherine Parr, took an interest in the high intelligence of Elizabeth, and supported the decision to provide her with an impressive education after Henry's death, starting when Elizabeth was 9.^[26] Elizabeth received an education equal to that of a prominent male aristocrat; she was educated in Latin, Greek, Spanish, French, philosophy, history, mathematics and music. England reaped the reward of her rich education when circumstances resulted in her becoming a capable monarch.

By the 18th century, Europeans recognized the value of literacy, and schools were opened to educate the public in growing numbers. Education in the Age of Enlightenment in France led to up to a third of women becoming literate by the time of the French Revolution, contrasting with roughly half of men by that time.^[27] However, education was still not considered as important for girls as for boys, who were being trained for professions that remained closed to women, and girls were not admitted to secondary level schools in France until the late 19th century. Girls were not entitled to receive a Baccalaureate diploma in France until the reforms of 1924 under education minister Léon Bérard. Schools were segregated in France until the end of World War II. Since then, compulsory education laws have raised the education of girls and young women throughout Europe.

"Coming of age" customs[link]

Many cultures have traditional customs to mark the "coming of age" of a girl or boy, to recognize their transition to adulthood, or to mark other milestones of their journey to maturity as children.

Japan has a coming-of-age ritual called Shichi-Go-San (七五三), which literally means "Seven-Five-Three". This is a traditional rite of passage and festival day in Japan for three- and seven-year-old girls and three- and five-year-old boys, held annually on November 15. It is generally observed on the nearest weekend. On this day, the girl will be dressed in a traditional kimono, and will be taken to a temple by her family for a blessing ceremony. Nowadays, the occasion is also marked with a formal photo portrait.

Some coming-of-age ceremonies are religious rituals to recognize a girl's maturity with respect to her understanding of religious beliefs, and to recognize her changing role in her religious community. Confirmation is a ceremony common to many Christian denominations for both boys and girls, usually taking place when the child is in their teen years. In Roman Catholic communities, Confirmation ceremonies are considered one of seven sacraments that a Catholic may receive during their life. In many countries, it is traditional for Catholics children to undergo another sacrament, First Communion, at the age of 7 years old. The sacrament is usually performed in a church once a year, with children who are of age receive a blessing from a Bishop in a special ceremony. It is traditional in many countries for Catholic girls to wear white dresses and possibly a small veil or wreath of flowers in their hair to their First Communion. The white dress symbolizes spiritual purity.

Many coming-of-age ceremonies are to acknowledge the passing of a girl through puberty, when she experiences menarche, or her first menstruation. The traditional Apache coming-of-age ceremony for girls is called the na'ii'ees (Sunrise Ceremony), and takes place over four days. The girls are painted with clay and pollen, which they must not wash off until the end of the rituals, which involve dancing and rituals that challenge physical strength. Girls are given teaching in aspects of sexuality, confidence, and healing ability. The girls pray in the direction of the east at dawn, and in the four cardinal directions, which represent the four stages of life. This ceremony was banned by the U.S. government for many decades; after being decriminalized by the Indian Religious Freedom Act in 1978, it has seen a revival. ^[28]

Preparing girls for marriage[link]

A postwar German girl helps her mother, who is struggling for income, prepare food to sell, 1946.

In many ancient societies, girls' upbringing had much to do with preparing them to be future wives. In many cultures, it was not the norm for women to be economically independent. Thus, where a girl's future well-being depended upon marrying her to a man who was economically self-sufficient, it was crucial to prepare her to meet whatever qualities or skills were popularly expected of wives.

In cultures ranging from Ancient Greece to the 19th century United States, girls have been taught such essential domestic skills as sewing, cooking, gardening, and basic hygiene and medical care such as preparing balms and salves, and in some cases midwife skills. These skills would be taught from generation to generation, with the knowledge passed down orally from mother to daughter. A well-known reference to these important women's skills is in the folk tale Rumpelstiltskin, which dates back to Medieval Germany and was collected in written form by the folklorists the Brothers Grimm. The miller's daughter is valued as a potential wife because of her reputation for being able to spin straw into gold.

In some parts of China, beginning in the Southern Tang kingdom in Nanjing (937-975), the custom of foot binding was associated with upper class women who were worthy of a life of leisure, and husbands who could afford to spare them the necessity of work (which would require the ability to be mobile and spend the day on their feet). Because of this belief, parents hoping to ensure a good marriage for their daughters would begin binding their feet from about the age of seven years old to achieve the ideal appearance. The tinier the feet, the better the social rank of a future husband. This practice did not end until the early years of the 20th century.^[29]

China has had many customs tied to girls and their roles as future wives and mothers. According to one custom, a girl's way of wearing her hair would indicate her marital status. An unmarried girl would wear her hair in two "pigtails", and once married, she would wear her hair in one.^[30]

In some cultures, girls' passing through puberty is viewed with concern for a girl's chastity. In some communities, there is a traditional belief that female genital mutilation is a necessity to prevent a girl from becoming sexually promiscuous. The practice is dangerous, however, and leads to long-term health problems for women who have undergone it. The practice has been a custom in 28 countries of Africa, and persists mainly in rural areas. This coming-of-age custom, sometimes incorrectly described as "female circumcision", is being outlawed by governments, and challenged by human rights groups and other concerned community members, who are working to end the practice.

Girls' games and other pastimes[link]

Girls' pastimes, such as games and other leisure activities, reflect the customs of their communities.

• 

  A girl applying henna art in Morocco.

• 

  A girl enjoying a game of table football in Mauritania.

See also[link]

Wikimedia Commons has media related to: Girls

Look up girl in Wiktionary, the free dictionary.

• Boy
• Female infanticide
• Girl group
• Girl Guides
• Girl Power
• Tomboy
• Woman

References[link]

Fall foliage splendor in the Green Mountain National Forest

Autumn or fall (/ˈɔːtəm/ AW-təm or /fɔːl/ fawl, respectively) is one of the four temperate seasons. Autumn marks the transition from summer into winter, in September (Northern Hemisphere) or March (Southern Hemisphere) when the arrival of night becomes noticeably earlier.

The equinoxes might be expected to be in the middle of their respective seasons, but temperature lag (caused by the thermal latency of the ground and sea) means that seasons appear later than dates calculated from a purely astronomical perspective. The actual lag varies with region. Some cultures regard the autumnal equinox as "mid-autumn", others with a longer lag treat it as the start of autumn.^[1] Meteorologists (and most of the temperate countries in the southern hemisphere)^[2] use a definition based on months, with autumn being September, October and November in the northern hemisphere,^[3] and March, April and May in the southern hemisphere.

In North America, autumn is usually considered to start with the September equinox.^[4] In traditional East Asian solar term, autumn starts on or around 8 August and ends on about 7 November. In Ireland, the autumn months according to the national meteorological service, Met Éireann, are September, October and November.^[5] However, according to the Irish Calendar which is based on ancient Gaelic traditions, autumn lasts throughout the months of August, September, and October, or possibly a few days later, depending on tradition. In Australia, autumn officially begins on March 1 and ends May 31^[6] According to United States tradition, autumn runs from the day after Labor Day (i.e. the Tuesday following the first Monday of September) through Thanksgiving (i.e. the fourth Thursday in November), after which the holiday season that demarcates the unofficial beginning of winter begins.

Etymology[link]

The word autumn comes from the Old French word autompne (automne in modern French), and was later normalised to the original Latin word autumnus.^[7] There are rare examples of its use as early as the 12th century, but it became common by the 16th century.

Before the 16th century, harvest was the term usually used to refer to the season, as it is common in other West Germanic languages to this day (cf. Dutch herfst and German Herbst). However, as more people gradually moved from working the land to living in towns (especially those who could read and write,  the only people whose use of language we now know), the word harvest lost its reference to the time of year and came to refer only to the actual activity of reaping, and autumn, as well as fall, began to replace it as a reference to the season.^[8][9]

The alternative word fall for the season traces its origins to old Germanic languages. The exact derivation is unclear, the Old English fiæll or feallan and the Old Norse fall all being possible candidates. However, these words all have the meaning "to fall from a height" and are clearly derived either from a common root or from each other. The term came to denote the season in 16th century England, a contraction of Middle English expressions like "fall of the leaf" and "fall of the year".^[10]

During the 17th century, English emigration to the British colonies in North America was at its peak, and the new settlers took the English language with them. While the term fall gradually became obsolete in Britain, it became the more common term in North America.^{[citation needed]}

Harvest association[link]

Association with the transition from warm to cold weather, and its related status as the season of the primary harvest, has dominated its themes and popular images. In Western cultures, personifications of autumn are usually pretty, well-fed females adorned with fruits, vegetables and grains that ripen at this time. Many cultures feature autumnal harvest festivals, often the most important on their calendars. Still extant echoes of these celebrations are found in the mid-autumn Thanksgiving holiday of the United States and Canada, and the Jewish Sukkot holiday with its roots as a full-moon harvest festival of "tabernacles" (huts wherein the harvest was processed and which later gained religious significance). There are also the many North American Indian festivals tied to harvest of autumnally ripe foods gathered in the wild, the Chinese Mid-Autumn or Moon festival, and many others. The predominant mood of these autumnal celebrations is a gladness for the fruits of the earth mixed with a certain melancholy linked to the imminent arrival of harsh weather.

This view is presented in English poet John Keats' poem To Autumn, where he describes the season as a time of bounteous fecundity, a time of 'mellow fruitfulness'.

While most foods are harvested during the autumn, foods particularly associated with the season include pumpkins (which are integral parts of both Thanksgiving and Halloween) and apples, which are used to make the seasonal beverage apple cider.

Melancholy association[link]

Autumn in New Hampshire

Autumn in poetry has often been associated with melancholy. The possibilities of summer are gone, and the chill of winter is on the horizon. Skies turn grey, and many people turn inward, both physically and mentally.^[11]

Similar examples may be found in Irish poet William Butler Yeats' poem The Wild Swans at Coole where the maturing season that the poet observes symbolically represents his own ageing self. Like the natural world that he observes he too has reached his prime and now must look forward to the inevitability of old age and death. French poet Paul Verlaine's "Chanson d'automne" ("Autumn Song") is likewise characterised by strong, painful feelings of sorrow. Keats' To Autumn, written in September 1819, echoes this sense of melancholic reflection, but also emphasises the lush abundance of the season.

Other associations[link]

Halloween pumpkins

In the Anglosphere, most notably in Anglo-America, autumn is also associated with the Halloween season (which in turn was influenced by Samhain, a Celtic autumn festival),^[12] and with it a widespread marketing campaign that promotes it, in the U.S.A. The television, film, book, costume, home decoration, and confectionery industries use this time of year to promote products closely associated with such a holiday, with promotions going from early September to 31 October, since their themes rapidly lose strength once the holiday ends, and advertising starts concentrating on Christmas.

Autumn has a strong association with American football, as the regular season begins during September and ends with playoff competition in December or January, in the winter season. Canadian football, on the other hand, begins in the summer, but extends its season through the autumn season and into November. A normal activity for high schools in the US is attending Friday night football games in Autumn, while Sunday afternoons are reserved for the professional game, particularly the National Football League, and Saturdays are traditionally used for college football. The sport is generally geared around fall weather and playing in cold elements.

Television stations and networks, particularly in North America, traditionally begin their regular seasons in autumn, with new series and new episodes of existing series debuting mostly during late September (series that debut outside the fall season are usually known as midseason replacements). A sweeps period takes place in November to measure Nielsen Ratings.

Since 1997, Autumn has been one of the top 100 names for girls in the United States.^[13]

In Indian mythology, autumn is considered to be the preferred season for the goddess of learning Saraswati, who is also known by the name of "goddess of autumn" (Sharada).

Tourism[link]

Although colour change in leaves occurs wherever deciduous trees are found, coloured autumn foliage is most famously noted in three regions of the world: most of Anglo-America, Eastern Asia (including China, Korea, and Japan), and Europe.

Eastern Canada and New England are famous for their autumnal foliage,^[14][15] and this attracts major tourism (worth billions of U.S. dollars) for the regions.^[16][17]

Paintings[link]

• 

  Otoño, Frederic Edwin Church, 1875. Museo Thyssen-Bornemisza^[18]

• 

  John Everett Millais, "Autumn Leaves".

• 

  Early Autumn, Qian Xuan, 13th century, depiction of decaying lotus leaves and dragonflies hovering over stagnant water

• 

  Autumn, Giuseppe Arcimboldo, 1573

Gallery[link]

• 

  Cherry trees in autumn

• 

  Oak tree in autumn

• 

  A brightly coloured tree contrasts with the green foliage which surrounds it

• 

  A forested hill over Struma (river) valey in Vitosha

• 

  Copper beech in autumn

• 

  Autumn season tree from Kuwait

References[link]

This article uses bare URLs for citations. Please consider adding full citations so that the article remains verifiable. Several templates and the Reflinks tool are available to assist in formatting. (Reflinks documentation) (February 2012)

Wikimedia Commons has media related to: Autumn

Wikiquote has a collection of quotations related to: Fall

 This article incorporates text from a publication now in the public domain: Chambers, Ephraim, ed. (1728). "^{article name needed}". Cyclopaedia, or an Universal Dictionary of Arts and Sciences (first ed.). James and John Knapton, et al. 

External links[link]

Look up autumn or fall in Wiktionary, the free dictionary.

• How To Photograph Autumn Color
• What Season Is It?

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Sleeping child

Sleep is a naturally recurring state characterized by reduced or absent consciousness, relatively suspended sensory activity, and inactivity of nearly all voluntary muscles.^[1] It is distinguished from quiet wakefulness by a decreased ability to react to stimuli, and is more easily reversible than being in hibernation or a coma. Sleep is a heightened anabolic state, accentuating the growth and rejuvenation of the immune, nervous, skeletal and muscular systems. It is observed in all mammals, all birds, and many reptiles, amphibians, and fish.

The purposes and mechanisms of sleep are only partially clear and are the subject of intense research.^[2] Sleep is often thought to help conserve energy,^[3][4] but decreases metabolism only about 5–10%.^[3][4] Hibernating animals need to sleep despite the hypometabolism seen in hibernation, and must return from hypothermia to euthermia in order to sleep, making sleeping "energetically expensive."^[5]

Physiology[link]

Sleep stages[link]

In mammals and birds, sleep is divided into two broad types: rapid eye movement (REM) and non-rapid eye movement (NREM or non-REM) sleep. Each type has a distinct set of associated physiological, neurological, and psychological features. The American Academy of Sleep Medicine (AASM) further divides NREM into three stages: N1, N2, and N3, the last of which is also called delta sleep or slow-wave sleep (SWS).^[6]

Hypnogram showing sleep cycles from midnight to 6.30 am, with deep sleep early on. There is more REM (marked red) before waking.

Stage N3 sleep; EEG highlighted by red box. Thirty seconds of deep sleep, here with greater than 50% delta waves.

REM sleep; EEG highlighted by red box; eye movements highlighted by red line. Thirty seconds of sleep.

Sleep proceeds in cycles of REM and NREM, the order normally being N1 → N2 → N3 → N2 → REM. There is a greater amount of deep sleep (stage N3) earlier in the sleep cycle, while the proportion of REM sleep increases later in the sleep cycle and just before natural awakening.

The stages of sleep were first described in 1937 by Alfred Lee Loomis and his coworkers, who separated the different electroencephalography (EEG) features of sleep into five levels (A to E), which represented the spectrum from wakefulness to deep sleep.^[7] In 1953, REM sleep was discovered as distinct, and thus William Dement and Nathaniel Kleitman reclassified sleep into four NREM stages and REM.^[8] The staging criteria were standardized in 1968 by Allan Rechtschaffen and Anthony Kales in the "R&K sleep scoring manual."^[9] In the R&K standard, NREM sleep was divided into four stages, with slow-wave sleep comprising stages 3 and 4. In stage 3, delta waves made up less than 50% of the total wave patterns, while they made up more than 50% in stage 4. Furthermore, REM sleep was sometimes referred to as stage 5.

In 2004, the AASM commissioned the AASM Visual Scoring Task Force to review the R&K scoring system. The review resulted in several changes, the most significant being the combination of stages 3 and 4 into Stage N3. The revised scoring was published in 2007 as The AASM Manual for the Scoring of Sleep and Associated Events.^[10] Arousals and respiratory, cardiac, and movement events were also added.^[11][12]

Sleep stages and other characteristics of sleep are commonly assessed by polysomnography in a specialized sleep laboratory. Measurements taken include EEG of brain waves, electrooculography (EOG) of eye movements, and electromyography (EMG) of skeletal muscle activity. In humans, the average length of the first sleep cycle is approximately 90 minutes and 100 to 120 minutes from the second to the fourth cycle, which is usually the last one.^[13] Each stage may have a distinct physiological function and this can result in sleep that exhibits loss of consciousness but does not fulfill its physiological functions (i.e., one may still feel tired after apparently sufficient sleep).

Scientific studies on sleep have shown that sleep stage at awakening is an important factor in amplifying sleep inertia. Alarm clocks involving sleep stage monitoring appeared on the market in 2005.^[14] Using sensing technologies such as EEG electrodes or accelerometers, these alarm clocks are supposed to wake people only from light sleep.

NREM sleep[link]

According to the 2007 AASM standards, NREM consists of three stages. There is relatively little dreaming in NREM.

Stage N1 refers to the transition of the brain from alpha waves having a frequency of 8–13 Hz (common in the awake state) to theta waves having a frequency of 4–7 Hz. This stage is sometimes referred to as somnolence or drowsy sleep. Sudden twitches and hypnic jerks, also known as positive myoclonus, may be associated with the onset of sleep during N1. Some people may also experience hypnagogic hallucinations during this stage. During N1, the subject loses some muscle tone and most conscious awareness of the external environment.

Stage N2 is characterized by sleep spindles ranging from 11 to 16 Hz (most commonly 12–14 Hz) and K-complexes. During this stage, muscular activity as measured by EMG decreases, and conscious awareness of the external environment disappears. This stage occupies 45–55% of total sleep in adults.

Stage N3 (deep or slow-wave sleep) is characterized by the presence of a minimum of 20% delta waves ranging from 0.5–2 Hz and having a peak-to-peak amplitude >75 μV. (EEG standards define delta waves to be from 0 to 4 Hz, but sleep standards in both the original R&K, as well as the new 2007 AASM guidelines have a range of 0.5–2 Hz.) This is the stage in which parasomnias such as night terrors, nocturnal enuresis, sleepwalking, and somniloquy occur. Many illustrations and descriptions still show a stage N3 with 20–50% delta waves and a stage N4 with greater than 50% delta waves; these have been combined as stage N3.

REM sleep[link]

Rapid eye movement sleep, or REM sleep (also known as paradoxical sleep), accounts for 20–25% of total sleep time in most human adults. The criteria for REM sleep include rapid eye movements as well as a rapid low-voltage EEG. Most memorable dreaming occurs in this stage. At least in mammals, a descending muscular atonia is seen. Such paralysis may be necessary to protect organisms from self-damage through physically acting out scenes from the often-vivid dreams that occur during this stage.

Timing[link]

The human biological clock

Sleep timing is controlled by the circadian clock, sleep-wake homeostasis, and in humans, within certain bounds, willed behavior. The circadian clock—an inner timekeeping, temperature-fluctuating, enzyme-controlling device—works in tandem with adenosine, a neurotransmitter that inhibits many of the bodily processes associated with wakefulness. Adenosine is created over the course of the day; high levels of adenosine lead to sleepiness^{[citation needed]}. In diurnal animals, sleepiness occurs as the circadian element causes the release of the hormone melatonin and a gradual decrease in core body temperature. The timing is affected by one's chronotype. It is the circadian rhythm that determines the ideal timing of a correctly structured and restorative sleep episode.^[15]

Homeostatic sleep propensity (the need for sleep as a function of the amount of time elapsed since the last adequate sleep episode) must be balanced against the circadian element for satisfactory sleep.^[16] Along with corresponding messages from the circadian clock, this tells the body it needs to sleep.^[17] Sleep offset (awakening) is primarily determined by circadian rhythm. A person who regularly awakens at an early hour will generally not be able to sleep much later than his or her normal waking time, even if moderately sleep-deprived.

Sleep duration is affected by the gene DEC2. Some people have a mutation of this gene; they sleep two hours less than normal. Neurology professor Ying-Hui Fu and her colleagues bred mice that carried the DEC2 mutation and slept less than normal mice.^[18][19]

Optimal amount in humans[link]

Adult[link]

The optimal amount of sleep is not a meaningful concept unless the timing of that sleep is seen in relation to an individual's circadian rhythms. A person's major sleep episode is relatively inefficient and inadequate when it occurs at the "wrong" time of day; one should be asleep at least six hours before the lowest body temperature.^[20] The timing is correct when the following two circadian markers occur after the middle of the sleep episode and before awakening:^[21]

• maximum concentration of the hormone melatonin, and
• minimum core body temperature.

For more information on the human circadian rhythm and body temperature, see Biological markers (in the article Circadian rhythm).

Human sleep needs can vary by age and among individuals, and sleep is considered to be adequate when there is no daytime sleepiness or dysfunction. Moreover, self-reported sleep duration is only moderately correlated with actual sleep time as measured by actigraphy,^[22] and those affected with sleep state misperception may typically report having slept only four hours despite having slept a full eight hours.^[23]

A University of California, San Diego psychiatry study of more than one million adults found that people who live the longest self-report sleeping for six to seven hours each night.^[24] Another study of sleep duration and mortality risk in women showed similar results.^[25] Other studies show that "sleeping more than 7 to 8 hours per day has been consistently associated with increased mortality," though this study suggests the cause is probably other factors such as depression and socioeconomic status, which would correlate statistically.^[26] It has been suggested that the correlation between lower sleep hours and reduced morbidity only occurs with those who wake naturally, rather than those who use an alarm.

Main health effects of sleep deprivation,^[27] indicating impairment of normal maintenance by sleep

Researchers at the University of Warwick and University College London have found that lack of sleep can more than double the risk of death from cardiovascular disease, but that too much sleep can also be associated with a doubling of the risk of death, though not primarily from cardiovascular disease.^[28][29] Professor Francesco Cappuccio said, "Short sleep has been shown to be a risk factor for weight gain, hypertension, and Type 2 diabetes, sometimes leading to mortality; but in contrast to the short sleep-mortality association, it appears that no potential mechanisms by which long sleep could be associated with increased mortality have yet been investigated. Some candidate causes for this include depression, low socioeconomic status, and cancer-related fatigue... In terms of prevention, our findings indicate that consistently sleeping around seven hours per night is optimal for health, and a sustained reduction may predispose to ill health."

Furthermore, sleep difficulties are closely associated with psychiatric disorders such as depression, alcoholism, and bipolar disorder.^[30] Up to 90% of adults with depression are found to have sleep difficulties. Dysregulation found on EEG includes disturbances in sleep continuity, decreased delta sleep and altered REM patterns with regard to latency, distribution across the night and density of eye movements.^[31]

Hours by age[link]

Children need more sleep per day in order to develop and function properly: up to 18 hours for newborn babies, with a declining rate as a child ages.^[17] A newborn baby spends almost 9 hours a day in REM sleep. By the age of five or so, only slightly over two hours is spent in REM. Studies say that school age children need about 10 to 11 hours of sleep.^[32]

Siesta or nap[link]

The siesta habit has recently been associated with a 37% reduction in coronary mortality, possibly due to reduced cardiovascular stress mediated by daytime sleep.^[35] Nevertheless, epidemiological studies on the relations between cardiovascular health and siesta have led to conflicting conclusions, possibly because of poor control of moderator variables, such as physical activity. It is possible that people who take a siesta have different physical activity habits, e.g., waking earlier and scheduling more activity during the morning. Such differences in physical activity may mediate different 24-hour profiles in cardiovascular function. Even if such effects of physical activity can be discounted for explaining the relationship between siesta and cardiovascular health, it is still unknown whether it is the daytime nap itself, a supine posture or the expectancy of a nap that is the most important factor. It was recently suggested that a short nap can reduce stress and blood pressure (BP), with the main changes in BP occurring between the time of lights off and the onset of stage 1.^[36][37] Dr. Zaregarizi and his team have conducted that the acute time of falling asleep was where beneficial cardiovascular changes take place. This study has indicated that a large decline in blood pressure occurs during the daytime sleep-onset period only when sleep is expected however when subjects rest in a supine position, the same reduction in blood pressure is not observed. This blood pressure reduction may be associated with the lower coronary mortality rates seen in Mediterranean and Latin American populations where siestas are common. Dr. Zaregarizi assessed cardiovascular function (blood pressure, heart rate, and measurements of blood vessel dilation) while nine healthy volunteers, 34 years of age on average, spent an hour standing quietly; reclining at rest but not sleeping; or reclining to nap. All participants were restricted to 4 hours of sleep on the night prior to each of the sleep laboratory tests. During the three phases of daytime sleep, he noted significant reductions in blood pressure and heart rate. By contrast, they did not observe changes in cardiovascular function while the participants were standing or reclining at rest. These findings also show that the greatest decline in blood pressure occurs between lights-off and onset of daytime sleep itself. During this sleep period, which lasted 9.7 minutes on average, blood pressure decreased, while blood vessel dilation increased by more than 9 percent. “There is little change in blood pressure once a subject is actually asleep," Dr. Zaregarizi noted, and he found minor changes in blood vessel dilation during sleep.^[36][37]

Sleep debt[link]

Sleep debt is the effect of not getting enough sleep; a large debt causes mental, emotional and physical fatigue.

Sleep debt results in diminished abilities to perform high-level cognitive functions. Neurophysiological and functional imaging studies have demonstrated that frontal regions of the brain are particularly responsive to homeostatic sleep pressure.^[38]

Scientists do not agree on how much sleep debt it is possible to accumulate; whether it is accumulated against an individual's average sleep or some other benchmark; nor on whether the prevalence of sleep debt among adults has changed appreciably in the industrialized world in recent decades. It is likely that children are sleeping less than previously in Western societies.^[39]

Genetics[link]

It is suspected that a considerable amount of sleep-related behavior, such as when and how long a person needs to sleep, is regulated by our genetics. Researchers have discovered some evidence that seems to support this assumption.^[40] ABCC9 is one gene found to be potentially significant.^{[citation needed]}

Functions[link]

A Kutchi woman sleeping

The multiple theories proposed to explain the function of sleep reflect the as-yet incomplete understanding of the subject. (When asked, after 50 years of research, what he knew about the reason people sleep William Dement, founder of Stanford University's Sleep Research Center, answered, "As far as I know, the only reason we need to sleep that is really, really solid is because we get sleepy."^[41]) It is likely that sleep evolved to fulfill some primeval function and took on multiple functions over time^{[citation needed]} (analogous to the larynx, which controls the passage of food and air, but descended over time to develop speech capabilities).

If sleep were not essential, one would expect to find:

• Animal species that do not sleep at all
• Animals that do not need recovery sleep when they stay awake longer than usual
• Animals that suffer no serious consequences as a result of lack of sleep

Outside of a few basal animals that have no brain or a very simple one, no animals have been found to date that satisfy any of these criteria.^[42] While some varieties of shark, such as great whites and hammerheads, must remain in motion at all times to move oxygenated water over their gills, it is possible they still sleep one cerebral hemisphere at a time as marine mammals do. However it remains to be shown definitively whether any fish is capable of unihemispheric sleep.

Some of the many proposed functions of sleep are as follows:

Restoration[link]

Wound healing has been shown to be affected by sleep. A study conducted by Gumustekin et al.^[43] in 2004 shows sleep deprivation hindering the healing of burns on rats.

It has been shown that sleep deprivation affects the immune system. In a study by Zager et al. in 2007,^[44] rats were deprived of sleep for 24 hours. When compared with a control group, the sleep-deprived rats' blood tests indicated a 20% decrease in white blood cell count, a significant change in the immune system. It is now possible to state that "sleep loss impairs immune function and immune challenge alters sleep," and it has been suggested that mammalian species which invest in longer sleep times are investing in the immune system, as species with the longer sleep times have higher white blood cell counts.^[45] Sleep has also been theorized to effectively combat the accumulation of free radicals in the brain, by increasing the efficiency of endogeneous antioxidant mechanisms.^[46]

It has yet to be proven that sleep duration affects somatic growth. One study by Jenni et al.^[47] in 2007 recorded growth, height, and weight, as correlated to parent-reported time in bed in 305 children over a period of nine years (age 1–10). It was found that "the variation of sleep duration among children does not seem to have an effect on growth." It has been shown that sleep—more specifically, slow-wave sleep (SWS)—does affect growth hormone levels in adult men. During eight hours' sleep, Van Cauter, Leproult, and Plat^[48] found that the men with a high percentage of SWS (average 24%) also had high growth hormone secretion, while subjects with a low percentage of SWS (average 9%) had low growth hormone secretion.

There are multiple arguments supporting the restorative function of sleep. The metabolic phase during sleep is anabolic; anabolic hormones such as growth hormones (as mentioned above) are secreted preferentially during sleep. The duration of sleep among species is, in general, inversely related to animal size^{[citation needed]} and directly related to basal metabolic rate. Rats with a very high basal metabolic rate sleep for up to 14 hours a day, whereas elephants and giraffes with lower BMRs sleep only 3–4 hours per day.

Energy conservation could as well have been accomplished by resting quiescent without shutting off the organism from the environment, potentially a dangerous situation. A sedentary nonsleeping animal is more likely to survive predators, while still preserving energy. Sleep, therefore, seems to serve another purpose, or other purposes, than simply conserving energy; for example, hibernating animals waking up from hibernation go into rebound sleep because of lack of sleep during the hibernation period. They are definitely well-rested and are conserving energy during hibernation, but need sleep for something else.^[5] Rats kept awake indefinitely develop skin lesions, hyperphagia, loss of body mass, hypothermia, and, eventually, fatal sepsis.^[49]

Ontogenesis[link]

According to the ontogenetic hypothesis of REM sleep, the activity occurring during neonatal REM sleep (or active sleep) seems to be particularly important to the developing organism (Marks et al., 1995). Studies investigating the effects of deprivation of active sleep have shown that deprivation early in life can result in behavioral problems, permanent sleep disruption, decreased brain mass (Mirmiran et al., 1983), and an abnormal amount of neuronal cell death.^[50]

REM sleep appears to be important for development of the brain. REM sleep occupies the majority of time of sleep of infants, who spend most of their time sleeping. Among different species, the more immature the baby is born, the more time it spends in REM sleep. Proponents also suggest that REM-induced muscle inhibition in the presence of brain activation exists to allow for brain development by activating the synapses, yet without any motor consequences that may get the infant in trouble. Additionally, REM deprivation results in developmental abnormalities later in life.

However, this does not explain why older adults still need REM sleep. Aquatic mammal infants do not have REM sleep in infancy;^[51] REM sleep in those animals increases as they age.

Memory processing[link]

Scientists have shown numerous ways in which sleep is related to memory. In a study conducted by Turner, Drummond, Salamat, and Brown,^[52] working memory was shown to be affected by sleep deprivation. Working memory is important because it keeps information active for further processing and supports higher-level cognitive functions such as decision making, reasoning, and episodic memory. The study allowed 18 women and 22 men to sleep only 26 minutes per night over a four-day period. Subjects were given initial cognitive tests while well-rested, and then were tested again twice a day during the four days of sleep deprivation. On the final test, the average working memory span of the sleep-deprived group had dropped by 38% in comparison to the control group.

Memory seems to be affected differently by certain stages of sleep such as REM and slow-wave sleep (SWS). In one study,^[53] multiple groups of human subjects were used: wake control groups and sleep test groups. Sleep and wake groups were taught a task and were then tested on it, both on early and late nights, with the order of nights balanced across participants. When the subjects' brains were scanned during sleep, hypnograms revealed that SWS was the dominant sleep stage during the early night, representing around 23% on average for sleep stage activity. The early-night test group performed 16% better on the declarative memory test than the control group. During late-night sleep, REM became the most active sleep stage at about 24%, and the late-night test group performed 25% better on the procedural memory test than the control group. This indicates that procedural memory benefits from late, REM-rich sleep, whereas declarative memory benefits from early, SWS-rich sleep.

A study conducted by Datta^[54] indirectly supports these results. The subjects chosen were 22 male rats. A box was constructed wherein a single rat could move freely from one end to the other. The bottom of the box was made of a steel grate. A light would shine in the box accompanied by a sound. After a five-second delay, an electrical shock would be applied. Once the shock commenced, the rat could move to the other end of the box, ending the shock immediately. The rat could also use the five-second delay to move to the other end of the box and avoid the shock entirely. The length of the shock never exceeded five seconds. This was repeated 30 times for half the rats. The other half, the control group, was placed in the same trial, but the rats were shocked regardless of their reaction. After each of the training sessions, the rat would be placed in a recording cage for six hours of polygraphic recordings. This process was repeated for three consecutive days. This study found that during the posttrial sleep recording session, rats spent 25.47% more time in REM sleep after learning trials than after control trials. These trials support the results of the Born et al. study, indicating an obvious correlation between REM sleep and procedural knowledge.

An observation of the Datta study is that the learning group spent 180% more time in SWS than did the control group during the post-trial sleep-recording session. This phenomenon is supported by a study performed by Kudrimoti, Barnes, and McNaughton.^[55] This study shows that after spatial exploration activity, patterns of hippocampal place cells are reactivated during SWS following the experiment. In a study by Kudrimoti et al., seven rats were run through a linear track using rewards on either end. The rats would then be placed in the track for 30 minutes to allow them to adjust (PRE), then they ran the track with reward-based training for 30 minutes (RUN), and then they were allowed to rest for 30 minutes. During each of these three periods, EEG data were collected for information on the rats' sleep stages. Kudrimoti et al. computed the mean firing rates of hippocampal place cells during prebehavior SWS (PRE) and three ten-minute intervals in postbehavior SWS (POST) by averaging across 22 track-running sessions from seven rats. The results showed that ten minutes after the trial RUN session, there was a 12% increase in the mean firing rate of hippocampal place cells from the PRE level; however, after 20 minutes, the mean firing rate returned rapidly toward the PRE level. The elevated firing of hippocampal place cells during SWS after spatial exploration could explain why there were elevated levels of SWS sleep in Datta's study, as it also dealt with a form of spatial exploration.

A study has also been done involving direct current stimulation to the prefrontal cortex to increase the amount of slow oscillations during SWSfe. The direct current stimulation greatly enhanced word-pair retention the following day, giving evidence that SWS plays a large role in the consolidation of episodic memories.^[56]

The different studies all suggest that there is a correlation between sleep and the complex functions of memory. Harvard sleep researchers Saper^[57] and Stickgold^[58] point out that an essential part of memory and learning consists of nerve cell dendrites' sending of information to the cell body to be organized into new neuronal connections. This process demands that no external information is presented to these dendrites, and it is suggested that this may be why it is during sleep that memories and knowledge are solidified and organized.

Preservation[link]

The "Preservation and Protection" theory holds that sleep serves an adaptive function. It protects the animal during that portion of the 24-hour day in which being awake, and hence roaming around, would place the individual at greatest risk.^[59] Organisms do not require 24 hours to feed themselves and meet other necessities. From this perspective of adaptation, organisms are safer by staying out of harm's way, where potentially they could be prey to other, stronger organisms. They sleep at times that maximize their safety, given their physical capacities and their habitats.

This theory fails to explain why the brain disengages from the external environment during normal sleep. However, the brain consumes a large proportion of the body's calories at any one time and preservation of energy could only occur by limiting its sensory inputs. Another argument against the theory is that sleep is not simply a passive consequence of removing the animal from the environment, but is a "drive"; animals alter their behaviors in order to obtain sleep. Therefore, circadian regulation is more than sufficient to explain periods of activity and quiescence that are adaptive to an organism, but the more peculiar specializations of sleep probably serve different and unknown functions. Moreover, the preservation theory needs to explain why carnivores like lions, which are on top of the food chain and thus have little to fear, sleep the most. It has been suggested that they need to minimize energy expenditure when not hunting.

Preservation also does not explain why aquatic mammals sleep while moving. Quiescence during these vulnerable hours would do the same and would be more advantageous, because the animal would still be able to respond to environmental challenges like predators, etc. Sleep rebound that occurs after a sleepless night will be maladaptive, but obviously must occur for a reason. A zebra falling asleep the day after it spent the sleeping time running from a lion is more, not less, vulnerable to predation.

Dreaming[link]

Bronze statue of Eros sleeping, 3rd century BC–early 1st century AD

Dreaming is the perceived experience of sensory images and sounds during sleep, in a sequence which the dreamer usually perceives more as an apparent participant than as an observer. Dreaming is stimulated by the pons and mostly occurs during the REM phase of sleep.

People have proposed many hypotheses about the functions of dreaming. Sigmund Freud postulated that dreams are the symbolic expression of frustrated desires that had been relegated to the unconscious mind, and he used dream interpretation in the form of psychoanalysis to uncover these desires. See Freud: The Interpretation of Dreams.

Freud's work concerns the psychological role of dreams, which does not exclude any physiological role they may have. Recent research^[60] claims that sleep has the overall role of consolidation and organization of synaptic connections formed during learning and experience. As such, Freud's work is not ruled out. Nevertheless, Freud's research has been expanded on, especially with regard to the organization and consolidation of recent memory.

Certain processes in the cerebral cortex have been studied by John Allan Hobson and Robert McCarley. In their activation synthesis theory, for example, they propose that dreams are caused by the random firing of neurons in the cerebral cortex during the REM period. Neatly, this theory helps explain the irrationality of the mind during REM periods, as, according to this theory, the forebrain then creates a story in an attempt to reconcile and make sense of the nonsensical sensory information presented to it.^[61] Ergo, the odd nature of many dreams.

Effect of food and drink on sleep[link]

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Hypnotics[link]

• Nonbenzodiazepine hypnotics such as eszopiclone (Lunesta), zaleplon (Sonata), and zolpidem (Ambien) are commonly used as sleep aids prescribed by doctors to treat forms of insomnia. Nonbenzodiazepines are the most commonly prescribed and OTC sleep aids used worldwide and have been greatly growing in use since the 1990s. They target the GABA_A receptor.
• Benzodiazepines target the GABA_A receptor also, and as such, they are commonly used sleep aids as well, though benzodiazepines have been found to decrease REM sleep.^[62]
• Antihistamines, such as diphenhydramine (Benadryl) and doxylamine (found in various OTC medicines, such as NyQuil)
• Alcohol – Often, people start drinking alcohol in order to get to sleep (alcohol is initially a sedative and will cause somnolence, encouraging sleep).^[63] However, being addicted to alcohol can lead to disrupted sleep, because alcohol has a rebound effect later in the night. As a result, there is strong evidence linking alcoholism and forms of insomnia.^[64] Alcohol also reduces REM sleep.^[62]
• Barbiturates cause drowsiness and have actions similar to alcohol in that they have a rebound effect and inhibit REM sleep, so they are not used as a long-term sleep aid.^[65]
• Melatonin is a naturally occurring hormone that regulates sleepiness. It is made in the brain, where tryptophan is converted into serotonin and then into melatonin, which is released at night by the pineal gland to induce and maintain sleep. Melatonin supplementation may be used as a sleep aid, both as a hypnotic and as a chronobiotic (see phase response curve, PRC).
• Siesta and the "post-lunch dip" – Many people have a temporary drop in alertness in the early afternoon, commonly known as the "post-lunch dip." While a large meal can make a person feel sleepy, the post-lunch dip is mostly an effect of the biological clock. People naturally feel most sleepy (have the greatest "drive for sleep") at two times of the day about 12 hours apart—for example, at 2:00 a.m. and 2:00 p.m. At those two times, the body clock "kicks in." At about 2 p.m. (14:00), it overrides the homeostatic buildup of sleep debt, allowing several more hours of wakefulness. At about 2 a.m. (02:00), with the daily sleep debt paid off, it "kicks in" again to ensure a few more hours of sleep.
• Tryptophan – The amino acid tryptophan is a building block of proteins. It has been claimed to contribute to sleepiness, since it is a precursor of the neurotransmitter serotonin, involved in sleep regulation. However, no solid data have ever linked modest dietary changes in tryptophan to changes in sleep.
• Marijuana – Some people use marijuana to induce sleepiness. Users often report relaxation and drowsiness. It has been shown that Tetrahydrocannabinol, the principal psychoactive constituent in marijuana, reduces the amount of REM sleep.^[66] Frequent users often report being unable to recall their dreams.

Stimulants[link]

• Amphetamine (dextroamphetamine, and a related, slightly more powerful drug methamphetamine, etc.) are used to treat narcolepsy. Their most common effects are anxiety, insomnia, stimulation, increased alertness, and decreased hunger.
• Caffeine is a stimulant that works by slowing the action of the hormones in the brain that cause somnolence, particularly by acting as an antagonist at adenosine receptors. Effective dosage is individual, in part dependent on prior usage. It can cause a rapid reduction in alertness as it wears off.
• Cocaine and crack cocaine – Studies on cocaine have shown its effects to be mediated through the circadian rhythm system.^[67] This may be related to the onset of hypersomnia (oversleeping) in regard to "cocaine-induced sleep disorder."^[68]
• MDMA, including similar drugs like MDA, MMDA, or bk-MDMA – The class of drugs called empathogen-entactogens keep users awake with intense euphoria. Commonly known as "ecstasy."
• Methylphenidate – Commonly known by the brand names Ritalin and Concerta, methylphenidate is similar in action to amphetamine and cocaine; its chemical composition more closely resembles that of cocaine.
• Tobacco – Tobacco has been found not only to disrupt but also to reduce total sleep time. In studies, users have described more daytime drowsiness than nonsmokers.^[69]
• Other analeptic drugs like Modafinil and Armodafinil are prescribed to treat narcolepsy, hypersomnia, shift work sleep disorder, and other conditions causing Excessive Daytime Sleepiness. The precise mechanism of these CNS stimulants is not known, but they have been shown to increase both the release of monoamines and levels of hypothalamic histamine, thereby promoting wakefulness.

Insomnia[link]

Insomnia is a general term describing difficulty falling asleep and staying asleep. Insomnia can have many different causes, including psychological stress, a poor sleep environment, an inconsistent sleep schedule, or excessive mental or physical stimulation in the hours before bedtime. Insomnia is often treated through behavioral changes like keeping a regular sleep schedule, avoiding stimulating or stressful activities before bedtime, and cutting down on stimulants such as caffeine. Patients are often counseled to improve their sleep environment by installing heavy drapes to shut out all sunlight, and keeping computers, televisions and work materials out of the sleeping area.

A 2010 review of published scientific research suggested that exercise generally improves sleep for most people, and helps sleep disorders such as insomnia. The optimum time to exercise may be 4 to 8 hours before bedtime, though exercise at any time of day is beneficial, with the exception of heavy exercise taken shortly before bedtime, which may disturb sleep. However there is insufficient evidence to draw detailed conclusions about the relationship between exercise and sleep.^[70]

Sleeping medications such as Ambien and Lunesta are an increasingly popular treatment for insomnia, and have become a major source of revenue for drug companies. Although these nonbenzodiazepine medications are generally believed to be better and safer than earlier generations of sedatives, they have still generated some controversy and discussion regarding side-effects.

White noise appears to be a promising treatment for insomnia.^[71]

Obstructive sleep apnea[link]

Obstructive sleep apnea is a condition in which major pauses in breathing occur during sleep, disrupting the normal progression of sleep and often causing other more severe health problems. Apneas occur when the muscles around the patient's airway relax during sleep, causing the airway to collapse and block the intake of oxygen. As oxygen levels in the blood drop, the patient then comes out of deep sleep in order to resume breathing. When several of these episodes occur per hour, sleep apnea rises to a level of seriousness that may require treatment.

Diagnosing sleep apnea usually requires a professional sleep study performed in a sleep clinic, because the episodes of wakefulness caused by the disorder are extremely brief and patients usually do not remember experiencing them. Instead, many patients simply feel tired after getting several hours of sleep and have no idea why. Major risk factors for sleep apnea include chronic fatigue, old age, obesity and snoring.

Other sleep disorders[link]

People sleeping on a train at night

Sleep disorders include narcolepsy, periodic limb movement disorder (PLMD), restless leg syndrome (RLS), and the circadian rhythm sleep disorders. Fatal familial insomnia, or FFI, is an extremely rare genetic disease with no known treatment or cure, is characterized by increasing insomnia as one of its symptoms; ultimately sufferers of the disease stop sleeping entirely, before dying of the disease.^[41]

Older people are more easily awakened by disturbances in the environment^[72] and may to some degree lose the ability to consolidate sleep.

Anthropology of sleep[link]

Research suggests that sleep patterns vary significantly across cultures.^[73][74] The most striking differences are between societies that have plentiful sources of artificial light and ones that do not.^[73] The primary difference appears to be that pre-light cultures have more broken-up sleep patterns.^[73] For example, people might go to sleep far sooner after the sun sets, but then wake up several times throughout the night, punctuating their sleep with periods of wakefulness, perhaps lasting several hours.^[73] The boundaries between sleeping and waking are blurred in these societies.^[73] Some observers believe that nighttime sleep in these societies is most often split into two main periods, the first characterized primarily by deep sleep and the second by REM sleep.^[73]

Some societies display a fragmented sleep pattern in which people sleep at all times of the day and night for shorter periods. In many nomadic or hunter-gatherer societies, people will sleep on and off throughout the day or night depending on what is happening.^[73] Plentiful artificial light has been available in the industrialized West since at least the mid-19th century, and sleep patterns have changed significantly everywhere that lighting has been introduced.^[73] In general, people sleep in a more concentrated burst through the night, going to sleep much later, although this is not always true.^[73] Historian Roger Ekrich thinks that the traditional pattern of "segmented sleep" as it is called began to disappear among the urban upper class in Europe in the late 17th century and the change spread over the next 200 years; by the 1920s "the idea of a first and second sleep had receded entirely from our social consciousness."^[75] Ekrich attributes the change to increases in "street lighting, domestic lighting and a surge in coffee houses," which slowly made nighttime a legitimate time for activity, decreasing the time available for rest.^[75]

In some societies, people generally sleep with at least one other person (sometimes many) or with animals. In other cultures, people rarely sleep with anyone but a most intimate relation, such as a spouse. In almost all societies, sleeping partners are strongly regulated by social standards. For example, people might only sleep with their immediate family, extended family, spouses, their children, children of a certain age, children of specific gender, peers of a certain gender, friends, peers of equal social rank, or with no one at all. Sleep may be an actively social time, depending on the sleep groupings, with no constraints on noise or activity.^[73]

People sleep in a variety of locations. Some sleep directly on the ground; others on a skin or blanket; others sleep on platforms or beds. Some sleep with blankets, some with pillows, some with simple headrests, some with no head support. These choices are shaped by a variety of factors, such as climate, protection from predators, housing type, technology, personal preference, and the incidence of pests.^[73]

Sleep in non-humans[link]

Sleeping Japanese Macaques

Neurological sleep states can be difficult to detect in some animals. In these cases, sleep may be defined using behavioral characteristics such as minimal movement, postures typical for the species, and reduced responsiveness to external stimulation. Sleep is quickly reversible, as opposed to hibernation or coma, and sleep deprivation is followed by longer or deeper rebound sleep. Herbivores, who require a long waking period to gather and consume their diet, typically sleep less each day than similarly sized carnivores, who might well consume several days' supply of meat in a sitting.

Horses and other herbivorous ungulates can sleep while standing, but must necessarily lie down for REM sleep (which causes muscular atony) for short periods. Giraffes, for example, only need to lie down for REM sleep for a few minutes at a time. Bats sleep while hanging upside down. Some aquatic mammals and some birds can sleep with one half of the brain while the other half is awake, so-called unihemispheric slow-wave sleep.^[76] Birds and mammals have cycles of non-REM and REM sleep (as described above for humans), though birds' cycles are much shorter and they do not lose muscle tone (go limp) to the extent that most mammals do.

Many mammals sleep for a large proportion of each 24-hour period when they are very young.^[77] However, killer whales and some dolphins do not sleep during the first month of life.^[78] Such differences may be explained by the ability of land-mammal newborns to be easily protected by parents while sleeping, while marine animals must, even while very young, be more continuously vigilant for predators.

See also[link]

• Cortisol awakening response
• Microsleep
• Morvan's syndrome
• Polyphasic sleep
• Power nap
• Segmented sleep
• Sleep architecture
• Sleep medicine
• Somnology
• Sudden infant death syndrome
• Sudden unexpected death syndrome

Positions, practices, and rituals[link]

• Sleeping positions
• Co-sleeping
• Hypnosis
• Meditation
• Neutral spine
• Sleep hygiene
• Yoga-nidra
• World Sleep Day

References[link]