Cataplexy is a medical condition where people become limp when they are exposed to spontaneous emotions such as laughter, fright, anger, or sadness. People who have cataplexy will sometimes see that some of their muscles suddenly fail them. Cataplexy often affects people who have narcolepsy. Narcolepsy is a sleep disorder. People with narcolepsy tend to suddenly fall asleep. They cannot control when they fall asleep. Cataplexy is sometimes confused with epilepsy, where a series of flashes or other stimuli cause superficially similar seizures.