[Pyramus]

I've worked on detecting sleep stages from accelerometer data and it should work in theory (loss of muscle tone) but it doesn't in practice (still there are muscle twitches).

You need more modalities like heart rate, and a very sensitive heart rate monitor (the one in my Xiaomi/Amazfit is nowhere near the quality needed).

On the other hand, you can make quite good guesses based on the recurring pattern of REM sleep (in healthy subjects). Because REM sleep is quite prevalent in the later stages, your accuracy when guessing will be quite high.