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Q. What's the difference between a nightmare and a night terror? We think our son is having one or the other when he sleeps.

A. Nightmares usually hit during the last third of the night (between 4 and 6 a.m.) in REM sleep, which is when we dream; night terrors tend to occur earlier in the night (between 1 and 3 a.m.) during the deep stages of non-REM sleep. It can be hard to tell the two apart, but there are certain clues that can help you distinguish between them:

• When a child is having a nightmare, he can usually be woken up and comforted. With night terrors, which can cause a kid to scream, sweat, and appear terrified, it's very hard, if not impossible, to wake and soothe him.

• Children may recall a nightmare, even if vaguely, but they usually have no memory of a night terror.

Night terrors happen far less often than nightmares, are most common in young children, especially boys, and may run in families. It's unknown what exactly causes them, but they can be triggered by stress, sleep deprivation, and illness (especially with a high fever). They usually disappear by adolescence, although adults can get them, too. If your son is having frequent nightmares or night terrors, talk with your doctor about what might be causing them so you all can get a better night's sleep.