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This data is pretty dated. Your brain compensates. There's something called REM rebound to where if you don't get enough sleep, your brain will offload as much REM as possible to the end of your sleep, filling in the missing and crucial sleep cycle.
How does your brain know when the end of your sleep is?
Is your brain a separate entity? You literally decide when to wake to with alarm. Or if you don’t set a time, it wakes you after x hours, after it’s backloaded all the REM.
If sleep deprivation gets too bad, it’ll completely knock you out to get as much REM in as it feels you need to function, alarm or no alarm.
Studying neurobiology/neuroendocrine systems is a great way to question who really is in charge.
It's a lot of work man. Just being human. A lot... of work...
Billions of years of evolution
So it just anticipates when my alarm is about to go off?
It could be you subconsciously thinking about it because of anxiety. It happens to me sometimes.
That keeps me from going back to sleep all the time. Like oh I have another hour? Then it's 30 minutes to fall back asleep and then that's only 30 minutes so better I just get up. I get tired by the end of the day a lot lol
Weird polyphasic sleep schedules take weeks to start working, so probably from patterns. Although if what op comment is true something else is happening too