A cool body temperature is necessary for initiating sleep.

“The other thing is keeping the room cool. The reason keeping the room cool is useful for getting into and staying asleep is that throughout the night there are phases of sleep where you are paralyzed so-called REM sleep. That's a healthy paralysis. So you presumably so you can't act out your dreams. But there are portions of the night where you can move. And one of the more important movements that you do in the middle of the night is put your hand out or your foot out or you take your face out from under the covers as a means to cool yourself.”