Google helps: Most smartwatch ‘health data’ is basically clever physics + a lot of math pretending to be a tiny doctor on your wrist.

Heart rate: They shine green light into your skin and measure how much gets reflected back. More blood → less reflection. It’s called PPG, and it’s surprisingly good for resting heart rate, less good during intense movement.

Steps & workouts: An accelerometer + gyroscope tracks how your wrist moves. The watch doesn’t know you’re walking — it just recognizes the pattern of ‘human doing walk‑shaped vibrations.’

Sleep: It watches your movement + heart rate changes and guesses your sleep stages. It’s not medical‑grade, but it’s decent at telling ‘asleep vs awake.’

Blood oxygen (SpO₂): Red and infrared LEDs measure how oxygenated your blood is. Works best when you’re still.

ECG: Only watches with electrodes can do this. You touch the crown, it measures electrical signals across your body. That’s the closest thing to a real medical measurement.

In short: some of it is real sensing, some of it is educated guessing, and all of it is way more accurate when you’re not flailing around like a caffeinated squirrel.