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Most smart rings shine light into your finger and count the bounces. The ones that do it well sample at 100 Hz or higher, use green and infrared LEDs in a specific geometry, and process the signal before it ever reaches a server.
The wrist is the worst place to measure heart rate during sleep. It is too thick for transmissive PPG, too prone to motion artifact, and too thermally unstable. The finger wins on every metric that matters for overnight biometrics.
