How Your Watch Tracks Sleep WITHOUT Reading Your Brain

Your watch tells you how long you spent in deep sleep. But it never reads your brain.

So how can a watch that cannot see your brainwaves claim to know your sleep stages?

Real sleep stages are defined by brainwaves, which a watch cannot measure. So it watches outside clues instead. All night it tracks two things: how much you move, using its accelerometer, and your heart, using the light sensor. Those signals shift with each stage. Deep sleep brings near stillness and a slow, steady heartbeat. Light sleep shows more movement and a livelier pulse. In REM, your body goes still but your heart rate turns irregular. An algorithm matches these patterns to the most likely stage, then stitches the night into a graph.

So your sleep chart is not a recording, it is an educated guess. Watches tell asleep from awake fairly well, but are far shakier at naming exact stages, because they are inferring your brain from your wrist.

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