How does a smartwatch measure heart rate?

The technique is called photoplethysmography, or PPG. Blood absorbs green light strongly, so when the heart beats and more blood flows through the wrist, more light is absorbed and less reflects back to the sensor. Light-sensitive photodiodes detect these tiny changes many times per second, and software turns the pattern into beats per minute. Motion, a loose strap, tattoos and cold skin can make readings less accurate. Some watches also include electrical sensors that record a single-lead ECG when you touch the crown or bezel.