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What Is Narcolepsy?

Narcolepsy (NAR-ko-lep-se) is a disorder that causes periods of extreme daytime sleepiness. The disorder also may cause muscle weakness.

Most people who have narcolepsy have trouble sleeping at night. Some people who have the disorder fall asleep suddenly, even if they're in the middle of talking, eating, or another activity.

Narcolepsy also can cause:

  • Cataplexy (KAT-ah-plek-se). This condition causes a sudden loss of muscle tone while you're awake. Muscle weakness can affect certain parts of your body or your whole body. For example, if cataplexy affects your hand, you may drop what you're holding. Strong emotions often trigger this weakness. It may last seconds or minutes.
  • Hallucinations (ha-lu-sih-NA-shuns). These vivid dreams occur while falling asleep or waking up.
  • Sleep paralysis (pah-RAL-ih-sis). This condition prevents you from moving or speaking while waking up and sometimes while falling asleep. Sleep paralysis usually goes away within a few minutes.


The two main phases of sleep are nonrapid eye movement (NREM) and rapid eye movement (REM). Most people are in the NREM phase when they first fall asleep. After about 90 minutes of sleep, most people go from NREM to REM sleep.

Dreaming occurs during the REM phase of sleep. During REM, your muscles normally become limp. This prevents you from acting out your dreams. (For more information about sleep cycles, go to the National Heart, Lung, and Blood Institute's "Your Guide to Healthy Sleep.")

People who have narcolepsy often fall into REM sleep quickly and wake up directly from it. As a result, they may have vivid dreams while falling asleep and waking up.

Hypocretin (hi-po-KREET-in), a chemical in the brain, helps promote wakefulness. Most people who have narcolepsy have low levels of this chemical. What causes these low levels isn't well understood.

Researchers think that certain factors may work together to cause a lack of hypocretin. These factors may include heredity, infections, brain injuries, and autoimmune disorders. (Autoimmune disorders occur if the body's immune system mistakenly attacks the body's cells and tissues.)


Narcolepsy symptoms usually begin during the teen or young adult years. People who have narcolepsy may find it hard to function at school, work, home, and in social situations because of extreme tiredness.

Narcolepsy has no cure, but medicines, lifestyle changes, and other therapies can improve symptoms. Research is ongoing on the causes of narcolepsy and new ways to treat it.

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November 01, 2010 Last Updated Icon

The NHLBI updates Health Topics articles on a biennial cycle based on a thorough review of research findings and new literature. The articles also are updated as needed if important new research is published. The date on each Health Topics article reflects when the content was originally posted or last revised.