Basically as far as I know it’s the circadian rhythm (kind of an internal clock I guess, that also works with +/- 24h) taking care of that.
While sleeping, some chemicals in your brain keep you sleepy (like Adenosine), and they gradually decrease during the night. The closer you get to waking up, the more cortisol will be produced (which makes you awake/alert).
Also the hormon melatonin plays a role I think, it gets produced and kept at a high enough level through the night to keep you asleep, as morning approaches, the production starts to cease.
This is a good as I can put it, just FYI, no native, no medical background. Hope it still helps a little, even if it’s probably too superficial/simplified.
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