system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: what forces us to wake up and how does it work?


You can wake up naturally or be forced to wake up by e.g. loud noise, pain, water.

How does it work? 

Bonus question: Why is it possible to cut your blood supply to your arm and not be woken up for hours? (I slept on my arm for 6 hours, felt no arm after waking up)
gpt
A part of your brain is always active even while you’re sleeping. That part can monitor for natural dangers, which a loud noise, pain, water would all be perceived as, and wake you up.

edit: for your bonus question, you’re not cutting off the blood supply to your arm when you sleep on it, you’re squeezing a nerve, but I’m not sure of the details so maybe someone else can take over that one!