During a night, there will be several cycles of non-REM and REM sleep. The cause is unknown but night terrors are commonly associated with periods of emotional tension , stress , or conflict. Night terror is similar to nightmares except that nightmares usually occur during REM sleep and include unpleasant or frightening dreams.
Nightmares are most common in the early morning.
If recurrent, nightmares and night terror may be associated with psychological disturbances or severe stress, especially in adults. Night terror occurs most often in preadolescent boys, although it can occur in girls and in adults. It is fairly common in children 3 to 5 years old, and incidence usually reduces greatly after that. It may run in families. The number of episodes usually decreases after age 10.