Can you explain the stages of sleep?

Can you explain the stages of sleep?

The way a normal night occurs is you go into drowsy sleep, then into light sleep, then very quickly into deep sleep. It was called slow wave sleep because that's the way it looked when you were looking at brain waves; or stages 3 and 4 sleep. Nowadays we've lumped it all into calling it deep sleep. You then lighten a little bit and you go into rapid eye movement sleep; dreaming sleep. You have some of that, and then you go back into deep sleep, then some more rapid eye movement sleep, then the light sleep generally. And then more rapid eye movement sleep. The sleep itself is punctuated by an ultradian rhythm; by a 90 minute rhythm. So roughly every 90 minutes you go into rapid eye movement sleep. So for the whole night you'd get something like; 25% of the night would be deep sleep, 25% of the night would be dreaming sleep, but half the night would be light sleep. But the way it would be punctuated would be this 90 minute cycle. Predominantly deep sleep at the beginning of the night; predominantly rapid eye movement or dreaming sleep at the end of the night.