A nightmare is a dream that occurs during the rapid eye movement (REM) period of the sleep cycle that results in feelings of strong terror, fear, distress, or extreme anxiety.

People tend to have nightmares in the later part of the night, and they often wake up and recall the content of the bad dream. Nightmares can be a normal reaction to stress, and some clinicians believe they help people in working through traumatic events.

Frequently having nightmares is considered a disorder when it affects a person’s life by impairing their work or social life. Repeated nightmare is defined more specifically as a series of nightmares with a recurring theme.