Night terror, « Terreur nocturne » in french, also known as sleep terror, is a sleep disorder, causing feelings of terror or dread. Night terrors tend to happen during periods of arousal from delta sleep, also known as slow-wave sleep. During the first half of a sleep cycle, delta sleep occurs most often, which indicates that people with more delta sleep activity are more prone to night terrors. However, they can also occur during daytime naps.
Night terrors have been known since ancient times, although it was impossible to differentiate them from nightmares until rapid eye movement was discovered. Night terrors can often be mistaken for confusional arousal or alien abduction. Night terrors are a largely unknown phenomenon.