Definition of Excessive Daytime Sleepiness

Reviewed on 10/21/2021

Excessive daytime sleepiness is a neurological disorder in which there is a sudden recurrent uncontrollable compulsion to sleep. Excessive daytime sleepiness is also known as narcolepsy.

The condition is often associated with the following:

  • Cataplexy -- a sudden loss of muscle tone and paralysis of voluntary muscles associated with a strong emotion
  • Sleep paralysis -- immobility of the body that occurs in the transition from sleep to wakefulness
  • Hypnagogic hallucinations -- pre-sleep dreams
  • Automatic behaviors -- such as, for example, doing something "automatically" and not remembering afterward how you did it

More than 100,000 Americans have excessive daytime sleepiness (narcolepsy). It strikes both males and females and affects people of all races.

The symptoms most commonly appear in a person's teens and early 20s. The disease can vary in severity. Some people with it have mild sleepiness or rare cataplexy (less than one episode per week). Other people may have moderate sleepiness or infrequent cataplexy (less than one episode a day). Still other people with the disorder may experience severe sleepiness or have severe cataplexy (with one or more episodes of cataplexy per day).

The basic cause of narcolepsy is not known. It is not a fatal disorder in itself but it can lead to fatalities. For example, a narcoleptic may fall asleep while driving.

Narcolepsy is usually treated with a medication to improve alertness and an antidepressant that helps control cataplexy.

Other names for the condition include hypnolepsy, sleeping disease, paroxysmal sleep, and Gelineau syndrome.


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Jameson, J. Larry, et al. Harrison's Principles of Internal Medicine, 20th Ed. New York: McGraw-Hill Education, 2018.

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