Anecdotally, when I get extremely consistent sleep, like a week or more of going to sleep in the same 5-15 minute window and having an 8 A.M. alarm, I start to

Anecdotally, when I get extremely consistent sleep, like a week or more of going to sleep in the same 5-15 minute window and having an 8 A.M. alarm, I start to wake up at like 7:58/7:59 am. I suspect it's just my 'biological clock' falling into a routine. The actual biological mechanisms responsible for this is a question for someone more educated than I am in that space. Notable anecdote factors: I never consume caffeine after 3pm, I use taped down double layer blackout curtains so there is virtually no change in light level, and I have no blue light / stimulating mental activity an hour before bed.