So the deeper stages of sleep are really those times of quiescence, you're really restoring your body and we have a few different stages of sleep.

So we go through in the beginning of the night, we go into the really deep stages of sleep and we actually cycle through. So, when you go down to the deep stage, then you go back up and you actually come into something called REM sleep, which is after about 90 minutes.

And you cycle throughout so that you do about five to six cycles throughout the night. And we spend more time in REM later on in the night than we do earlier on.

So, more times than not, but not every time, it can be linked to a medical problem, such as menopause, cancer, chronic pain, it can be linked to anxiety and depression. Those are the more common causes.

There's some debate as to whether you need to awaken from them because there are some patients who are actually starting to say, "I had these horrible nightmares, but I never woke up from them." But they can still recall them when they get up in the morning. So there's still some debate in the field.

So when you're in REM sleep, your brain is very active, our body is quiet, but your brain is really processing a lot of things, a lot of emotions; we dream the most in REM sleep. And then you go back down in the deep stages, and so on and so forth.

Night terrors are in deep sleep, and they're more common in kids, as are nightmares, but what happens in a night terror is like a flash - we think a flash of some image or something happens in the brain. We don't really quite know what. And it usually ends up with the child screaming almost like screaming bloody murder. It's very scary for the parents or whoever else is around.