How does a smartwatch measure heart rate?
The green lights on the back of the watch shine into your skin hundreds of times per second. Blood soaks up green light, and with every heartbeat a little more blood pushes through your wrist. A light sensor measures how much green light bounces back, and the rhythmic dips reveal your pulse.
Green light shines into the skin
On the back of the watch, small green LEDs flash hundreds of times per second. The light goes a few millimeters into your wrist, where tiny blood vessels run just under the skin.
Share this stepEach heartbeat absorbs more light
Blood looks red because it soaks up most other colors of light, including green, and bounces back mainly red. So the more blood there is, the less green light comes back. With every heartbeat a small wave of blood swells the vessels, and the returning green light dips a little.
Share this stepA sensor counts the rhythm
Light sensors next to the LEDs measure the reflected light. The software looks for the regular rise and fall, filters out arm movements and turns the rhythm into beats per minute.
Share this stepYour watch counts heartbeats with light.
Green lightpulsing bloodheart rate
The method is called photoplethysmography, or PPG. It’s usually accurate at rest, but fast movements, a loose strap, cold hands and some tattoos can disturb it. Hospitals use a similar clip on the finger.
Common follow-up questions
Why is the light green?
Because blood soaks up green light so well, the changes with each heartbeat show up clearly. Red light would mostly bounce back no matter how much blood there is. Many watches also use infrared light, especially at night when green would be distracting.
How accurate is a smartwatch heart rate?
Close to a chest strap when you’re still or walking. During intense, jerky exercise the readings can lag or jump.
Can a smartwatch detect heart problems?
Some can flag an irregular rhythm or take a simple ECG and are approved for that use. They can’t rule out heart disease. Contact a doctor if you have symptoms, whatever the watch says.
What is a normal resting heart rate?
For most adults, 60 to 100 beats per minute at rest. Fit people are often lower.
