Nightmares tend to occur most often during rapid eye movement (REM) sleep, when most dreaming takes place. Because periods of REM sleep become progressively longer as the night progresses, you may find you experience nightmares most often in the early morning hours.
Parasomnias are disruptive sleep disorders that can occur during arousals from REM sleep or partial arousals from non-REM sleep. Parasomnias include nightmares, night terrors, sleepwalking, confusional arousals, and many others. Nightmares. Nightmares are vivid nighttime events that can cause feelings of fear, terror, and/or anxiety.