Sensor placement effect

Where a wearable sits can matter more than which device you buy. Both CIRQA reviews confirm it independently: rob-ter-horst measured biceps > wrist across every exercise (weightlifting 0.91 vs 0.73 correlation) and — surprisingly — for sleep staging too (possible mechanism: fewer degrees of freedom → less motion artifact → cleaner optical signal); dc-rainmaker adds the universal truism "if you move to the bicep, no matter what the optical sensor is, it will do better during workouts — conversely, during sleep it'll tend to do worse" (bicep wearables misjudge sleep times when your hands move but your bicep doesn't). The two reviewers agree on exercise, diverge on sleep — an open question worth tracking.

Sources: Quantified Scientist, DC Rainmaker