Hypersomnia is a disorder characterized by excessive sleepiness. There are two main categories of hypersomnia: primary hypersomnia (also called idiopathic hypersomnia) and recurrent hypersomnia (also called primary recurrent hypersomnia). Both have the same symptoms, but differ in frequency of occurrence.

Those who suffer from hypersomnia have recurring episodes of excessive daytime sleepiness (EDS), which is different from feeling tired due to lack of or interrupted sleep at night. They are compelled to nap repeatedly during the day, often at inappropriate times such as at work, during a meal, or in conversation. These daytime naps usually provide no relief from symptoms.

Patients with hypersomnia often experience prolonged night sleep and have difficulty waking from long sleep, feeling disoriented upon doing so. This condition is known as sleep drunkenness. Other symptoms may include anxiety, increased irritation, decreased energy, restlessness, slow thinking, slow speech, loss of appetite, hallucinations, and memory difficulty. Some patients lose the ability to function in family, social, occupational or other settings. Typically, hypersomnia is first recognized in adolescence or young adulthood. These symptoms are present in both types of hypersomnia. A sufferer of primary hypersomnia displays these symptoms continually for months or even years. Recurrent hypersomnia is characterized by recurring periods of symptoms many times throughout the year mixed with periods of normal sleep-wake cycles. Kleine-Levin syndrome is the most well-known form of recurrent hypersomnia, though it is very rare; sufferers often sleep up to eighteen hours a day and yet do not feel refreshed upon waking.