Nightmares are vividly realistic, disturbing dreams that rattle you awake from a deep sleep. They often set your heart pounding from fear. 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. The subjects of nightmares vary from person to person. There are, though, some common nightmares that many people experience. For example, a lot of adults have nightmares about not being able to run fast enough to escape danger or about falling from a great height. If you've gone through a traumatic event, such as an attack or accident, you may have recurrent nightmares about your experience. Although nightmares and night terrors both cause people to awake in great fear, they are different. Night terrors typically occur in the first few hours after falling asleep. They are experienced as feelings, not dreams, so people do not recall why they are terrified upon awakening.