How can you determine the state and phase of sleep?

Question

Health and computer

Good afternoon.
Recently, all sorts of fitness bracelets have appeared. My xiaomi band 2 can determine that the cell is sleeping and the sleep phase.
What are the reliable ways to determine that a person has fallen asleep (without encephalograms and tons of wires)? How can you reliably know the phase of sleep? Accelerometer, pulse, what else... Maybe there are other ways..

Answer 1

Enough accelerometer.
The accelerometer is in the "still" state - it means that you are sleeping deeply. "Mobile", but lying down (do not move meters in space) - fast sleep. And the response frequency of the accelerometer determines the difference between "a person is sleeping and turned on the other side" and "a person is lying and watching a movie."