The exact cause of narcolepsy is unknown. Studies have indicated that the disorder may be genetic, with studies showing gene markers that may indicate a tendency to develop narcolepsy. A small group of neurons in the brain has been implicated in producing transitions from sleep to wakefulness and vice-versa, and people with narcolepsy may have fewer of these neurons or they may have been damaged.

The condition may be aggravated by conditions that cause insomnia, such as disruption of work schedules.

Narcolepsy is characterized by episodes of frequent, uncontrollable daytime sleeping, usually preceded by drowsiness. The episodes usually occur after meals, but sudden onset of sleep may occur while working or driving a vehicle, having a conversation, or being in any sedentary or nonstimulating situation.

There is a brief period of sleep, and the person awakens feeling refreshed. However, he ar she may again become uncontrollably sleepy a short time later.

Narcolepsy can also be associated with cataplexy, a brief episode of severe loss of tone of various muscles. Generalized weakness may occur for a few moments during the transition between sleep and wake (sleep-paralysis). Many people with narcolepsy also have dreamlike hallucinations in the transition between sleep and wakefulness.