It seems that we need so much sleep - until we are satiated - and then no more (and perhaps that more than enough may be somewhat worse than enough: sleep obesity in effect!).
So in the early part of the night we have cycles of deep sleep, then - presumably - when they have done their work we move onto cycles of dreaming sleep; until, when that has done its work, we awake spontaneously.
The general function of deep sleep is probably strengthening of specific memories, the function of dreaming sleep editing (testing for coherence) narratives of memories.
Presumably, our brains know when these functions have been achieved to a sufficient degree: then we wake-up!
We are satiated with sleep: 'full-up' with sleep.