Da una pagina di supporto di Apple.com:
Throughout the day, Apple Watch measures your heart rate every 10 minutes and stores it in the Health app. All this information, as well as other data it collects, helps Apple Watch estimate how many calories you’ve burned. And by checking your heart rate during workouts, you can see how both your intensity level and your heart rate change over time.
The heart rate sensor in Apple Watch uses what is known as photoplethysmography. This technology, while difficult to pronounce, is based on a very simple fact: Blood is red because it reflects red light and absorbs green light. Apple Watch uses green LED lights paired with light‑sensitive photodiodes to detect the amount of blood flowing through your wrist at any given moment. When your heart beats, the blood flow in your wrist — and the green light absorption — is greater. Between beats, it’s less. By flashing its LED lights hundreds of times per second, Apple Watch can calculate the number of times the heart beats each minute — your heart rate.