Nightmare disorder is a sleep disorder characterized by frequent awakening from nightmares with a vivid remembrance of the dream. It is often caused by a major stressful event or from the use of SSRIs, such as Prozac and Effexor. Most cases reported begin at the age of 10 after a stressful life event. The frequency of the episodes is varied form person to person; generally one will suffer from three or four episodes every week.

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http://en.wikipedia.org/wiki/Nightmare_disorder