A sleep disorder whose primary symptom is irresistible attacks of sleepiness during the daytime.

Narcolepsy, which usually begins in adolescence or early adulthood, affects about one in every 1,000 persons and is equally common in males and females. The sleep attacks, which can occur anywhere from six to 20 times a day, usually last about 10 to 20 minutes but can persist for as long as two to three hours. Narcolepsy is diagnosed if sleep attacks occur every day for at least three months (although most people treated for the disorder suffer from it for a much longer period of time—often years—before seeking help). In addition to the sleep attacks, persons suffering from narcolepsy often display several other characteristic symptoms. The most debilitating of these is cataplexy, a sudden loss of muscle tone that can affect a part or all of the body. Cataplectic attacks range from a sagging jaw...