Sleep paralysis is something that is actually very common. Many people have it, I've had it myself. And what happens is, when you're in that REM stage of sleep, your brain is very active. You're dreaming your most during that stage, you're mind, your eyes are moving, there's a lot going on. It's like fireworks going on in your brain.

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 it's like your brain has a large filing cabinet and it's opening up each drawer and it's taking in various images and memories from the day, consolidating what it needs to and puts in whatever file. And then if there's something that doesn't fit in any of the files and doesn't really belong, you'll forget about it. So it's a way of really getting a succinct way of storing things in your brain.

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.

So when you go to sleep at night, if you're someone who hasn't had any sleep deprivation, you have a very normal sleep pattern, what we tend to see is that, in adults, they go to bed and they start off by going into the deeper stages sleep.

I'll work on patient's thoughts about sleep, "So I must get eight hours of sleep tonight or I won't sleep tomorrow." That sometimes - or "I won't function tomorrow." That sometimes makes it very difficult for you to sleep at night

Really, a nightmare just really has to evoke some sort of, we call it, dysphoric emotion or something uncomfortable. You could be sad, you could be unhappy; you could be scared, anxious. But traditionally, the definition is you have to awaken from this nightmare.