The distribution of sleep stages, with deep sleep occurring mostly in the first half of the night and REM sleep in the second half, is driven by duration awake and circadian time, but the functional evolutionary reason for this distribution is not understood.

“we have no idea why sleep plays out in the way in which it does that i described which is that you get most of your deep sleep in the first half of the night and most of your rem sleep in the second half of the night what i mean by that is we understand what determines why you get your deep sleep early in the night which is largely determined by how long you've been awake and a little bit on the basis of your circadian rhythm whereas rem sleep is much strongly much more powerfully determined by the circadian time of night but we still don't understand what the function of that skewed seesaw balance actually is”