How do smart rings monitor sleep?

Although everyone's sleep state is different, the state of human sleep is regular, and the main judgment of the hand ring is based on the motion variables from the stages of sleep to sleep to waking.

Every person from bed to real sleep, there will always be a variety of changes. The bracelet is worn on the wrist and is bound to produce a variety of displacement with the change of the body movement. The gravity change, the hand ring will record and judge whether the person is sleeping.

So, how can deep sleep and shallow sleep be judged, the same, when people are in deep sleep, they basically do not produce exercise, and the amount of exercise changes longer.