Narcolepsy is a disabling disorder of sleep regulation that affects the control of sleep and wakefulness. It may be described as an intrusion of the dream sleep (called REM or rapid eye movement) into the waking state. Symptoms generally begin between the ages of 15 and 30. The four classic symptoms of the disorder are excessive daytime sleepiness; cataplexy (sudden, brief episodes of muscle weakness or paralysis brought on by strong emotions such as laughter, anger, surprise or anticipation); sleep paralysis (paralysis upon falling asleep or waking up); and hypnagogic hallucinations (vivid dreamlike images that occur at sleep onset). Disturbed nighttime sleep, including tossing and turning in bed, leg jerks, nightmares, and frequent awakenings, may also occur. The development, number and severity of symptoms vary widely among individuals with the disorder. There appears to be an important genetic component to the disorder as well.