Your smartwatch is a good heart-rate monitor, a passable sleep tracker, and a poor calorie estimator — and it does one medical screening job surprisingly well That is the short version of what validation studies say about the numbers on your wrist. The longer version matters more, because every reading comes with fine print: how it was measured, under what conditions, and how much confidence it deserves. Researchers have now put the big brands through lab-grade testing — against chest-strap ECGs, hospital polysomnography, and metabolic carts — and the pattern is consistent. Here is the sensor-by-sensor breakdown. Heart rate: the measurement wearables do best Most watches read your pulse with photoplethysmography (PPG): green LEDs shine into your skin, and a sensor tracks how the reflected light changes as blood pulses through. It is an indirect measurement, which is why conditions matter so much. In a Stanford study of 60 volunteers wearing seven devices, six of the seven measured...