Health A-Z

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Harvard Medical School

Narcolepsy

What Is It?

Narcolepsy is a disorder that causes sudden episodes of deep sleep. These episodes can occur often and at inappropriate times, for example while a person is talking, eating or driving. Although sleep episodes can happen at any time, they may be more frequent during periods of inactivity or monotonous, repetitive activity.

Narcolepsy usually appears between ages 15 and 30, but the condition can appear earlier or later. Once it appears, narcolepsy is present for life. Men and women are affected equally. Narcolepsy affects at least 120,000 people in the United States.

About 60% of people with diagnosed narcolepsy have the combination of marked daytime sleepiness and sudden episodes of muscle weakness (called cataplexy). The muscle weakness can be so severe that a person with narcolepsy will collapse to the floor, but not become unconscious. This type of narcolepsy is associated with a shortage of a brain-stimulating protein called orexin (also known as hypocretin).

The cause of other types of narcolepsy is unknown. A genetic (inherited) predisposition appears to play a role.

People with narcolepsy don't require extra hours of sleep, but they do need daytime naps because they have difficulty staying awake for long periods. During the night, healthy people normally progress through several stages of sleep before entering or leaving the state of sleep called rapid eye movement (REM). During REM sleep, your brain waves resemble those of an awake person, visual dreams occur and muscle tone is slack. In narcolepsy, the brain-wave pattern can skip some or all of the other sleep stages, causing the person to move from the awake state immediately to REM sleep, or to awaken directly from the REM sleep stage.