Most nightmares are a normal reaction to stress, and some clinicians believe they help people work through traumatic events. Frequent occurrence of nightmares, however, becomes a disorder when it impairs social, occupational, and other important areas of functioning. At this point, it may be referred to as Nightmare Disorder (formerly Dream Anxiety Disorder) or "repeated nightmares," which is a term defined more specifically as a series of nightmares with a recurring theme.
Nightmares, which tend to be more common in girls than boys, usually begin in childhood before age 10 and are considered normal unless they significantly interfere with sleep, development, or psychosocial development. They may continue into adulthood where they are often associated with outside stressors, anxiety, or trauma. They may also exist alongside another mental disorder.