A frightening dream that occurs during REM (rapid eye movement) sleep.

Nightmares—frightening dreams—are the most common type of sleep disturbance in early childhood. They are distinguished from night terrors (pavor nocturnus), another childhood sleep disturbance, by the fact that night terrors occur during a different phase of sleep and do not involve dreaming. Nightmares are thought to be caused by a central nervous system response, and are related to other parasomnias such as sleepwalking.

Nightmares begin between the ages of 18 months and three years and increase in frequency and intensity around the ages of four and five years. Children this age have an exceptionally vivid fantasy life that carries over into their sleep. Their nightmares are typically characterized by feelings of danger and helplessness and often involve fleeing from monsters or wild animals. It is not unusual for a normal child this age to have nightmares...