Nightmares can be defined as vivid and terrifying dreams which awaken the dreamer from sleep. Typically, the dreamer wakes from the rapid eye movement (REM) stage of sleep and can remember a detailed, perhaps bizarre dream plot. There is evidence that frequent nightmares are associated with insomnia and poor-quality sleep.During the course of a nightmare the sleeper may moan, talk, or move slightly, although these signs do not always appear. The person wakes from the nightmare with a profound sense of fear.
Waking is complete, and usually accompanied by increased heart rate, sweating, and other symptoms of anxiety or fear. Once fully awake, the person usually has a good recall of the dream and what was so frightening about it. Because of the physical symptoms of anxiety and because clarity is achieved immediately upon waking, returning to sleep after a nightmare is often difficult. The vividness of the recall and the prominence of the dream images in the person's mind can also make it difficult to calm down and return to sleep.