[nottorp]

My theory is that your subconscious knows you have to wake up at a fixed time and that's extra stress, even if it wakes you up before the alarm. As you're saying, it even cuts into your sleep time.

One point in favor of this theory is that even I, a night owl, manage sometimes to wake up before the alarm on the rare days that I do need to wake up early on.

[geewee]

It happens even on prolonged vacations. So I don't actually have to get up at any particular time (my partner does) so I don't really buy the stress theory, it feels more like a pretty fixed circadian rhythm in regards to when I'm tired/awake - as I can push the rhythm but it takes a fair chunk of days. I imagine like adjusting to a new timezone.