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.

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

Well, the actual function of the brain, not so sure yet. There's a lot of different theories about it, but when you talk about psychologically in your brain, a lot of people with insomnia, though not all, report that they can't turn their minds off.

For some people they say, it's about wish fulfillment, it's about the things you are never able to do in your day you are actually fulfilling at night. There are other people who will say that it's actually telling you something.

So you have it, you awaken from it and you can recall, in detail, what just happened, that's a nightmare. So it's very different from a dream where you generally don't wake up from it and you don't have this dysphoric emotion.

It's uncommon, but there are some people who just have a delayed circadian rhythm and they just - they sleep better during the day then they do at night. So they've - a lot of those people with delayed sleep phase disorder they start to work in bars, they work some of the late night shifts, they sort of adjust to doing it more and more as time goes on.

So when it comes to circadian rhythms, it's a clock that's basically programmed in our body. So if you think back to times when people lived on farms and we didn't have electricity.