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Your smart ring does not measure sleep stages. It measures heart rate and motion, then guesses. Here's how the guess works, why the industry pretends otherwise, and why Pulsyn tells you the truth.
Most smart rings treat a 20-minute afternoon nap as either deep sleep or a complete miss. The reason is not a bug. It is a fundamental mismatch between how actigraphy guesses sleep stages and how naps actually work.
Skin temperature is a proxy for blood flow, circadian phase, and autonomic tone. Most smart rings average it into a single nightly number. We do not.
