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Smartwatches have evolved from simple timekeepers into powerful health monitors.
At the core of this change is an optical sensor that uses light to track changes in blood volume just beneath the skin.
This light‑sensing method lets your watch count each heartbeat without wires or chest straps.
1. How light reveals your pulse
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An illustration of a smartwatch tracking heart rate
Every time your heart pumps, extra blood fills the tiny vessels in your wrist.
Smartwatches shine a light onto your skin.
Blood absorbs green light more than the surrounding tissue, so when the heart pumps, less light is reflected to the watch's sensor.
A light detector beneath the watch face measures these tiny changes.
By spotting the regular pattern of rises and falls in reflected light, the watch calculates your beats per minute.
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2. Why green light works best
Green light emitted from a smartwatch sensor
Green LEDs are chosen for two reasons.
First, haemoglobin absorbs green wavelengths more strongly, producing a clearer signal.
Second, green LEDs require less power than infrared LEDs at equivalent brightness, helping to preserve battery life. Some devices also incorporate infrared sensors to support measurements in low‑light conditions or to estimate blood oxygen saturation.
3. The sensor’s design and position
Inside the watch you will find several green LEDs, one or more light detectors (photodiodes) and a small circuit board.
These parts sit under a transparent cover on the underside of the watch.
The underside of a smartwatch with sensors showing
A snug fit against the skin blocks out stray light and keeps readings steady.
Manufacturers recommend wearing the strap just above the wrist bone, tight enough to stay in place but still comfortable.
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4. Cutting out 'noise'
Raw sensor data can be noisy.
Simple movements, such as walking or shifting your arm, and changes in room lighting can cause false readings.
A person wearing a smartwatch
To clean up the signal, the watch uses digital filters to block out light fluctuations that are too slow or too fast to be heartbeats.
More advanced models compare readings from different LEDs and detectors to filter out motion. Some also use basic learning algorithms to improve accuracy during exercise.
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5. How accurate are the readings?
A smartwatch worn on the wrist
Although consumer smartwatches are not medical devices, they are surprisingly reliable.
Studies compare their readings with those from hospital‑grade electrocardiographs (ECGs).
At rest, the error rate is often below 5 per cent. During moderate exercise, it stays under 10 per cent.
Accuracy can vary by skin tone, watch fit and the device’s software.
Darker skin can absorb more light, which may slightly affect some models. Ongoing research aims to make readings accurate for everyone.
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6. Beyond heart rate
Heart rate monitoring is just the start.
Many watches now flag irregular rhythms that could signal atrial fibrillation (an irregular heartbeat).
A person wearing a smartwatch
By adding red and infrared lights, they also estimate blood‑oxygen levels, by assessing the ratio of oxygenated to deoxygenated haemoglobin.
Future updates may include features such as blood‑pressure estimates based on the speed of the blood pulse and even non‑invasive glucose monitoring.
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What does the future look like?
Thanks to simple light sensors and clever signal processing, smartwatches offer almost continuous heart‑rate monitoring on your wrist.
As the technology improves, these devices will continue to deliver deeper insights into your cardiovascular health without the need for needles or straps.