Nightmares occur during the dream phase of sleep known as REM sleep. Most people enter the REM stage of sleep sometime after 90 minutes of sleep. The circumstances of the nightmare will frighten the sleeper,who usually will wake up with a vivid memory of a long movie-like dream. Night terrors, on the other hand, occur during a phase of deep non-REM sleep usually within an hour after the subject goes to bed. This is also known as stage 4. (A link to a sleep stages chart can be found on the navigation bar to the left) During a night terror,which may last anywhere from five to twenty minutes, the person is still asleep. The sleeperseyes may be open, but they are not awake. When the subject does wake up, the subject usually has no recollection of the episode other than a sense of fear. This however is not always the case. Quite a few people interviewed can remember portions of the night terror, some remember the whole thing.