The disorder is often associated with 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), what are called hypnagogic hallucinations (pre-sleep dreams) and automatic behaviors (such as doing something "automatically" and not remembering afterwards how one did it).
125,000 Americans are estimated to have narcolepsy. It strikes males and females and all races. Symptoms most commonly appear in a person's teens and early twenties. The disease can vary in severity. Some persons with it have mild sleepiness or rare cataplexy (less than one episode per week). Other persons may have moderate sleepiness or infrequent cataplexy (less than one episode a day). Still other persons with narcolepsy may experience severe sleepiness or have severe cataplexy (with one or more episodes of cataplexy per day).
The cause(s) of narcolepsy are unknown. It is not a fatal disorder in itself but it can lead to fatalities. For example, a narcoleptic may fall asleep while driving.