We just learned about the Circadian Clock.
Another part of sleep is Non-Rapid Eye Movement Sleep.
We learned before about Rapid Eye Movement Sleep that happens when you are in deep sleep.
Non-Rapid Eye Movement Sleep is also called NREM, or quiescent sleep, and this is the part where you are falling asleep but aren't in deep sleep yet.
(from: wikipedia - non-rapid eye movement sleep)
Kid Facts - Blast from the past: Organ of Corti
