Narcolepsy is a sleep disorder characterized by excessive daytime sleepiness and sudden, uncontrollable sleep attacks. People with the condition may fall asleep at inappropriate times—during conversation, while driving, or in the middle of an activity.

Many also experience cataplexy, a sudden loss of muscle tone triggered by strong emotion, and some have sleep paralysis or vivid hypnagogic hallucinations at the edges of sleep. The disorder is linked to the loss of neurons in the hypothalamus that produce the neurotransmitter hypocretin (orexin), which helps maintain wakefulness. Onset typically occurs in adolescence or young adulthood.

Diagnosis relies on sleep history, overnight polysomnography, and a multiple sleep latency test that measures how quickly the person enters REM sleep during daytime naps. Treatment includes stimulant medications to promote alertness, drugs that reduce cataplexy, and behavioral strategies such as scheduled naps. Narcolepsy is lifelong, yet symptoms can often be managed sufficiently to allow work and social participation.

When the boundary between wakefulness and sleep becomes unstable, narcolepsy is the clinical expression of that instability.