Apple Watch tracks your heart rate using green LED lights on the back of the watch that flash hundreds of times per second against your skin. This technique, called photoplethysmography, detects tiny changes in blood flow through your wrist to calculate how many times your heart beats per minute. Some models also include an electrical heart sensor that works differently, used specifically for electrocardiogram (ECG) readings.
How the Optical Sensor Works
Blood is red because it absorbs green light. Apple Watch exploits this by shining green LEDs into your skin and measuring how much light bounces back using photodiodes (light detectors) built into the crystal on the back of the case. Each time your heart beats, blood surges through the small vessels in your wrist, absorbing more green light. Between beats, less blood is present, so more light reflects back. The watch reads these fluctuations in reflected light and translates the pattern into a heart rate number.
This is the same basic principle used in hospital pulse oximeters that clip onto your finger, just miniaturized and adapted for the wrist, where blood flow is weaker and movement is constant.
The Electrical Sensor for ECG
Starting with the Series 4, Apple Watch added an electrical heart sensor built into the digital crown and the back crystal. When you place your finger on the crown, a small electrical circuit completes across your chest. This picks up the tiny electrical signals your heart generates with each beat, producing a single-lead ECG similar to what you’d get from one lead in a clinical electrocardiogram.
The optical sensor and electrical sensor serve different purposes. The optical sensor tracks your heart rate continuously throughout the day. The electrical sensor only activates when you deliberately take an ECG reading, and its primary job is detecting irregular heart rhythms rather than simply counting beats per minute.
How It Filters Out Arm Movement
The biggest challenge for any wrist-based heart rate monitor is motion. When you swing your arm during a run, blood shifts around in your wrist and the watch can physically bounce against your skin, creating noise in the light signal that has nothing to do with your heartbeat.
Apple Watch handles this with its built-in accelerometer, a sensor that measures movement in three dimensions. The accelerometer data serves as a reference for what your arm is doing at any given moment. The watch’s processor then uses that reference to mathematically separate motion-related signal distortion from the actual pulse signal. This technique, known as adaptive noise cancellation, is a well-established method in wearable sensors. The accelerometer calculates the combined force of movement across all three axes, and algorithms subtract that motion pattern from the light sensor data to isolate the true heart rhythm.
During workouts, the watch also increases its LED flashing rate and cranks up the brightness of the green lights to push more signal through the noise. This is why you’ll notice the green lights glowing more intensely during exercise than when you’re sitting at a desk.
When and How Often It Measures
Apple Watch doesn’t monitor your heart rate every second of the day. It takes background readings throughout the day when you’re still, and periodically while walking. The time between these background measurements varies depending on your activity level. If you’re sitting quietly, readings happen roughly every few minutes. If you’re moving around, the intervals shift.
During an active workout session, the watch switches to continuous monitoring, sampling your heart rate every few seconds for a much more detailed picture. You can also force an on-demand reading at any time by opening the Heart Rate app.
Heart Rate Alerts and What Triggers Them
Apple Watch can notify you if your heart rate goes unusually high or low while you appear to be inactive. The default low heart rate threshold is set at 40 beats per minute. You can customize the high heart rate threshold, with options typically starting at 100 bpm for resting alerts and going up from there. These alerts only fire when the watch detects a sustained elevated or depressed rate, not from a single outlier reading.
The irregular rhythm notification feature works separately. It periodically checks your pulse pattern using the optical sensor and looks for signs of atrial fibrillation, a common type of irregular heartbeat. If it detects irregularity across multiple readings, it sends an alert.
What Affects Accuracy
Fit matters more than most people realize. The sensor needs consistent contact with your skin to get reliable readings. If the watch is too loose, ambient light leaks in and blood flow readings become erratic. Apple recommends wearing the band snug but comfortable, especially during exercise.
Tattoos are the most significant skin-related factor that can interfere with readings. Tattoo ink is opaque, which prevents the green light from penetrating through to the blood vessels beneath. Solid-colored tattoos, particularly red ones, absorb green light in a way that mimics blood absorption, confusing the sensor. Black tattoos absorb both green and red light, which can block the signal entirely. If you have a wrist tattoo under the sensor, you may get inconsistent or missing readings.
Interestingly, darker skin tones, scars, and skin abrasions do not meaningfully interfere with the sensor. Unlike tattoo ink, skin is translucent enough for the LED light to pass through regardless of pigmentation.
Cold weather can also reduce accuracy. When your body is cold, it constricts blood vessels in your extremities to preserve core heat, reducing blood flow through the wrist. Less blood flow means a weaker signal for the optical sensor to read. Certain activities with intense wrist flexion or vibration, like cycling on rough terrain, can also push the limits of what the motion-cancellation algorithms can handle.
How to Get the Best Readings
- Wear it correctly: Position the watch face on top of your wrist with the band firm enough that the sensor stays flat against your skin. During workouts, tighten it one notch.
- Keep the sensor clean: Sweat, lotion, and debris on the back crystal can scatter light and degrade signal quality. A quick wipe before a workout helps.
- Stay warm: If you’re exercising in cold conditions and notice erratic readings, warming your hands briefly can restore blood flow to the wrist.
- Check tattoo placement: If you have a tattoo where the sensor sits, consider wearing the watch on your other wrist or using a chest strap heart rate monitor that pairs via Bluetooth.