How The Apple Watch Monitors Your Heart Rate


One of the Apple Watch’s coolest features is its ability to monitor your heart rate. The watch monitors your heart rate throughout the day, and makes it available via Heart Rate Glance on the watch, and keeps track of your heart rate continuously while you’re doing a workout with the watch.

Apple is able to monitor your heart rate through a process called photoplethysmography, which uses light in an interesting way to determine your heart rate.

Specifically, since your blood is red, it reflects  red light and absorbs green light. To measure you heart rate, Apple Watch uses green LEDs along wit light-sensitive photodiodes to determine how much blood happens to be flowing through your wrist at a particular time. As your heart beats, more light is absorbed. Between heart beats, less light is absorbed. Apple Watch is flashing LEDs on your wrist hundreds of times er second, allowing it to get a pretty accurate idea of what your heart rate is at a given time.

The Apple Watch monitors your heart rate throughout the day in 10-minute increments. It then makes your heart rate available via the Glance functionality. 

If you’ve been using your Apple Watch to monitor your heart rate, you may have noticed that version 1.0.1 of Apple’s Apple Watch software seemed to change how your heart rate was being recorded. Specifically, it appeared to capture your heart rate less frequently.

Some users thought the missing records were a bug, but Apple has recently updated it support page to explain that it’s intentional.

Apple’s still monitoring your heart rate with the Apple Watch every 10 minutes, with one big exception: it won’t try to get your heart rate if your arm is moving.  Apple hasn’t announced why it made the change, however, some speculate it was to conserve battery power in the watch.

You can still force it to monitor your heart rate continuously by starting a workout.

In order to get an accurate reading, it’s important that your Apple Watch fit your wrist appropriately. If the watch is too loose, then it won’t be able to get a good reading on your wrist. Some users have also reported issues when putting the Apple Watch over tattooed skin. Since the Apple Watch is using light to determine your heart rate, tattooed skin can throw those sensors off and make it difficult to get an accurate reading or any reading at all. Apple has a number of other factors listed on its Apple Watch support page that can contribute to an inaccurate reading:

“A fancy way of describing how much blood flows through your skin, skin perfusion varies significantly from person to person and can also be impacted by the environment. If you’re exercising in the cold, for example, the skin perfusion in your wrist may be too low for the heart rate sensor to get a reading.

Motion is another factor that can affect the heart rate sensor. Rhythmic movements, such as running or cycling, give better results compared to irregular movements, like tennis or boxing.

Permanent or temporary changes to your skin, such as some tattoos, can also impact heart rate sensor performance.

The ink, pattern, and saturation of some tattoos can block light from the sensor, making it difficult to get reliable readings.”