You look at your smartwatch and see 72 bpm. But have you ever wondered how a small device sitting on your wrist actually knows your pulse rate? The answer involves light, blood flow and some fairly clever signal processing.
It starts with light
Most smartwatches use a technology called Photoplethysmography (PPG) to measure your pulse. The sensors on the back of the watch shine light into your skin. As your heart pumps blood, the amount of blood in the tiny blood vessels under your skin changes slightly with every pulse. These changes affect how much light is absorbed and reflected back to the watch. The sensor detects these tiny changes and creates a signal that follows the pattern of your pulse.
From a signal to a number
Your watch doesn't simply "count" your pulse. It first receives a continuously changing signal from your wrist. The device then uses algorithms to identify the repeated peaks in that signal and measures the time between them. From this information, it calculates your pulse rate in beats per minute (bpm).
So the number you see on your screen is actually the final result of several steps: Light → PPG signal → Signal processing → Pulse detection → Pulse rate.
Why isn't the reading always perfect?
Your body isn't a laboratory - and your wrist certainly isn't perfectly still. When you're exercising, your arm is moving while blood flow is changing at the same time. This can introduce additional signals into the measurement, known as motion artefacts. Other factors can also affect the quality of the signal, including:
- How tightly the watch fits
- Movement of the wrist
- Skin contact with the sensor
- Cold hands or reduced peripheral blood flow
- Sweat and moisture
- Position of the watch
Modern devices use algorithms to identify and reduce some of this noise, but no sensor can produce a perfect signal under every condition.
PPG and ECG are not the same thing
This is an important distinction. PPG uses light to detect changes associated with blood volume in the peripheral circulation and can be used to estimate your pulse rate. ECG (electrocardiography) records the electrical activity of the heart using electrodes.
So although both can tell us something about cardiac activity, they are measuring different physiological signals. Some smartwatches can perform an ECG when the appropriate sensors and features are available - but the ECG function should not be confused with the optical PPG sensor used for routine pulse-rate tracking.
And what about all the other numbers?
Your smartwatch may show you much more than pulse rate - such as SpO₂, HRV, sleep metrics, calories, VO₂ max and activity levels. But here's something worth remembering: not every number on your smartwatch is a direct measurement.
Some values come directly from a sensor, while others are calculated or estimated using algorithms, sometimes combining several signals and your personal information. The accuracy can therefore vary depending on the measurement, the device and the conditions in which it is being recorded.
So, should you trust your smartwatch?
Smartwatches can be very useful tools for tracking patterns and trends. A change that repeatedly appears over days or weeks may be much more informative than one isolated reading. But a smartwatch reading should always be understood in context. If your watch shows an unusual number, consider:
- What was being measured?
- How was it measured?
- Were you moving?
- Was the sensor making good contact?
- Is this a measurement or an estimate?
- And most importantly - how do you actually feel?
The bigger picture
Wearable technology is becoming an increasingly useful part of health and fitness monitoring. But understanding the technology makes it even more useful. Don't just look at the number on your wrist. Understand the signal behind it.
Clinical references
Primary professional guidance used to support the clinical context in this article.
