Circadian rhythm

A circadian rhythm /sɜːrˈkeɪdiən/ is any biological process that displays an endogenous, entrainable oscillation of about 24 hours. These 24-hour rhythms are driven by a circadian clock, and they have been widely observed in plants, animals, fungi, and cyanobacteria.

The term circadian comes from the Latin circa, meaning "around" (or "approximately"), and diēm, meaning "day". The formal study of biological temporal rhythms, such as daily, tidal, weekly, seasonal, and annual rhythms, is called chronobiology.

Although circadian rhythms are endogenous ("built-in", self-sustained), they are adjusted (entrained) to the local environment by external cues called zeitgebers (from German, "time giver"), which include light, temperature and redox cycles.

History

The earliest recorded account of a circadian process dates from the 4th century B.C.E., when Androsthenes, a ship captain serving under Alexander the Great, described diurnal leaf movements of the tamarind tree. The observation of a circadian or diurnal process in humans is mentioned in Chinese medical texts dated to around the 13th century, including the Noon and Midnight Manual and the Mnemonic Rhyme to Aid in the Selection of Acu-points According to the Diurnal Cycle, the Day of the Month and the Season of the Year.

Sleep

Sleep is a naturally recurring state of mind characterized by altered consciousness, relatively inhibited sensory activity, inhibition of nearly all voluntary muscles, and reduced interactions with surroundings. It is distinguished from wakefulness by a decreased ability to react to stimuli, but is more easily reversed than the state of hibernation or of being comatose. Mammalian sleep occurs in repeating periods, in which the body alternates between two highly distinct modes known as non-REM and REM sleep. REM stands for "rapid eye movement" but involves many other aspects including virtual paralysis of the body.

During sleep, most systems in an animal are in an anabolic state, building up the immune, nervous, skeletal, and muscular systems. Sleep in non-human animals is observed in mammals, birds, reptiles, amphibians, and fish, and, in some form, in insects and even in simpler animals such as nematodes. The internal circadian clock promotes sleep daily at night in diurnal species (such as humans) and in the day in nocturnal organisms (such as rodents). However, sleep patterns vary widely among animals and among different individual humans. Industrialization and artificial light have substantially altered human sleep habits in the last 100 years.

Khan Academy

Khan Academy is a non-profit educational organization created in 2006 by educator Salman Khan with the aim of providing a free, world-class education for anyone, anywhere. The organization produces short lectures in the form of YouTube videos. In addition to micro lectures, the organization's website features practice exercises and tools for educators. All resources are available for free to anyone around the world. The main language of the website is English, but the content is also available in other languages.

History

The founder of the organization, Salman Khan, was born in New Orleans, Louisiana, United States to immigrant parents from Bangladesh and India. After earning three degrees from the Massachusetts Institute of Technology (a BS in mathematics, a BS in electrical engineering and computer science, and an MEng in electrical engineering and computer science), he pursued an MBA from Harvard Business School.

In late 2004, Khan began tutoring his cousin Nadia who needed help with math using Yahoo!'s Doodle notepad.When other relatives and friends sought similar help, he decided that it would be more practical to distribute the tutorials on YouTube. The videos' popularity and the testimonials of appreciative students prompted Khan to quit his job in finance as a hedge fund analyst at Connective Capital Management in 2009, and focus on the tutorials (then released under the moniker "Khan Academy") full-time.

Lucid dream

A lucid dream is any dream during which the dreamer is aware that he or she is dreaming.

During lucid dreaming, the dreamer may allegedly be able to exert some degree of control over the dream characters, narrative, and environment.

The term 'lucid dream' was coined by Dutch author and psychiatrist Frederik van Eeden in his 1913 article A Study of Dreams, though descriptions of dreamers being aware that they are dreaming predates the term, and is closely related to ancient meditative praxis originating in India.

History

Western history

Ancient

The earliest references to a phenomenon comparable to that now signified by the term 'lucid dream' In Western culture are found in ancient Greek writings. For example, the philosopher Aristotle wrote: 'often when one is asleep, there is something in consciousness which declares that what then presents itself is but a dream'. Meanwhile, the physician Galen of Pergamon used lucid dreams as a form of therapy. In addition, a letter written by St. Augustine of Hippo in 415 AD tells the story of a dreamer, Doctor Gennadius, and refers to lucid dreaming.

Astral projection

Astral projection (or astral travel) is an interpretation of an out-of-body experience (OBE) that assumes the existence of an "astral body" separate from the physical body and capable of travelling outside it. Astral projection or travel denotes the astral body leaving the physical body to travel in an astral plane. The idea of astral travel is rooted in common worldwide religious accounts of the afterlife in which the consciousness' or soul's journey, or "ascent", is described in such terms as "an... out-of-body experience, wherein the spiritual traveller leaves the physical body and travels in his/her subtle body (or "dreambody", or "astral body") into ‘higher’ realms." It is frequently reported in association with dreams, and forms of meditation.

Some patients have reported perceptions similar to the descriptions of astral projection, induced through various hallucinogenic and hypnotic (including self-hypnotic) means. There is no scientific evidence that there is any manifestation of a consciousness or soul which is separate from neural activity. There is no scientific evidence that one can consciously leave the body and make observations. Claims of scientific evidence of astral projection are pseudoscientific.