How Apple Watch Measures Your Heart Rate

Apple has published a new support page detailing how the Apple Watch measures your heart rate. We currently know that the heart rate sensor helps more accurately track things like calories burned, but now Apple explains in detail the technology an software behind the heart rate sensor:

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.

Apple says Apple Watch measures your heart rate every 10 minutes. The data is then stored in the Health App included in iOS 8 where you can view later and also integrates with third-party Health and Fitness apps.

Apple also gives tips on how to wear your Apple Watch, suggesting that “your Apple Watch should be snug but comfortable” in order to receive the most accurate readings.

Finally, Apple mentions how Apple Watch is able to use different technologies to measure your activity depending on your location. For example, if you’re running indoors, the Watch uses its built-in accelerometer, but if you’re cycling outdoors, the Watch will use the GPS on your iPhone.

You can read more about the Apple Watch’s health functions here.