Apple has published a technical document describing how the Apple Watch heart rate sensor works. This sensor, together with movement data, helps the Apple Watch, and the Health app, determine how many calories you’ve burned. The document explains that the sensor uses infrared lights to measure the amount of blood flowing through your veins every ten minutes, and can also use green LEDs if the infrared doesn’t give an accurate reading.
However, it points out that you need to wear the watch fairly tight for these sensors to work. I had assumed this would be the case, as you need to do this with any fitness trackers that measure your heart rate, but when I tried on the watch at an Apple Store, the employee said that you can leave a finger’s width between your wrist and the watch.
Apple says that would be too loose, and that you need a snug fit. This can be problematic for people who want to use the Apple Watch as a fitness tracker, yet find a tight fit uncomfortable. This is certainly the case for some fitness trackers. Personally, I hope to use the Apple Watch as a fitness tracker, but I don’t want to wear it very tight. We’ll know more when the Apple Watch is available just how tight this needs to be.