How does a smart watch knows when people fall asleep, and having either light, deep or REM sleep, when it is only in contact with the wrist?

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How does a smart watch knows when people fall asleep, and having either light, deep or REM sleep, when it is only in contact with the wrist?

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By your heart rate. They have a heart rate sensor on the underside and depending on the rate it determines which state you are in. Not really much more complicated than that.

Smart watches have tons of sensors built in to measure data about its surroundings.

The data collected is paired with other sleep data to determine what stages of sleep your in. Movement heart rate, a light in your room or not etc.