system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: how does a smart watch know what phase of sleep you're in?

i imagine it tells rest from sleep by heart rate but how would it know if I'm in REM, or "deep sleep" or whatever other phases it says. are they ALL distinguishable by heart rate alone?

how legit is this readout anyway?
gpt
Your heart rate drops when you are in deep sleep (compared to light) and raises when you are in REM. It might also look at movement and (depending on your watch) skin temperature/blood-oxygen levels which change as well.

It's not as legit as, say, a sleep study where they stick sensors to your head to read your brain waves, but it's fairly accurate for what it is.