How much can you trust smartwatch health readings?
A smartwatch can help track activity and patterns in heart rate or sleep. Accuracy varies by device, feature, fit and circumstances, and some health features are estimates based on several sensor signals. An alert may be useful to bring to your doctor, but a reassuring reading does not rule out a medical problem.
The sensors inside a smartwatch
Almost everything a smartwatch reports comes from two parts. The first is an optical sensor on the underside. It shines green or red light into the skin and measures how much bounces back. Blood absorbs light, and the amount of blood under the sensor rises and falls with each heartbeat, so the returning light flickers in time with the pulse. This method is called photoplethysmography, and it is the same idea as the clip placed on a fingertip at the polyclinic, done with less precision from the wrist.
The second is an accelerometer, a tiny motion sensor that records how the wrist moves in three directions. Steps, activity minutes, and much of what the watch calls sleep come from this sensor.
Some models add electrodes for a short electrical recording of the heart's rhythm, a temperature sensor against the skin, and a satellite receiver for distance. Everything else on the screen is software's calculation from these raw signals, and that is where the guessing begins.
What the different readings can tell you
Heart rate at rest. Many watches give a fair estimate of pulse when the wrist is still and the fit is correct. Accuracy differs by model and person. Do not take reassurance from a reading that conflicts with your symptoms.
- Heart rate during exercise. Accuracy falls when the arms are moving, gripping or sweating. Readings can lag and drop out during rowing, weight training or cycling on rough ground. Steady running and brisk walking on a park connector tend to track well.
- Step count. For ordinary walking the count is reasonable. It undercounts when the hands are still, such as pushing a pram or a supermarket trolley, and overcounts when the wrist moves without walking, such as chopping vegetables. The National Steps Challenge treats the count as a motivation tool, which is the right level of trust.
- Heart-rate variability. This is the small variation in the gap between beats, estimated from the optical pulse rather than an electrical trace. The number is noisy from night to night. A trend over weeks can reflect recovery, illness or stress, and a single value means little.
- Sleep. The watch gives a fair estimate of total time asleep from stillness and heart rate. The division into light, deep and rapid-eye-movement sleep is a guess built on those same signals. A sleep laboratory measures brain waves, and a wrist cannot.
- Blood oxygen. The watch estimates oxygen saturation from red and infrared light. Movement, a loose fit and cold skin all disturb it, and overnight dips on the screen are often measurement artefacts. A medical pulse oximeter is a different instrument.
- The rhythm recording. The electrode feature records a single-lead trace of the heart's electrical activity. It can flag an irregular rhythm such as atrial fibrillation for a doctor to confirm. It cannot detect a heart attack, a blocked artery or most other heart problems.
Factors that affect accuracy
Follow the fit instructions for your model. As a rule, optical sensors need steady skin contact, and a loose strap or movement can disturb readings. Adjusting the fit can help, but it will not eliminate every source of error.
Skin contact, tattoos, skin temperature and pigmentation can affect optical measurements. Findings differ by device and by the measurement being studied. Do not assume that results about medical oxygen sensors apply in the same way to every watch’s heart-rate feature. Check your model’s limitations.
Cold hands narrow the blood vessels near the surface and weaken the signal. Sweat in Singapore's humidity can make the strap slide. Wrist movement is the largest single source of error, which is why the readings taken during sleep are the cleanest the watch collects.
Comparing readings over time
Comparing measurements made under similar conditions can make patterns easier to interpret. A sustained change in resting heart rate, sleep or exercise tolerance may be worth discussing. Do not dismiss repeated low oxygen readings, above all if you feel unwell.
Different features have different levels of validation. Step counts and resting heart rate can be useful, and some rhythm features are evaluated for specific medical uses. The manufacturer calculates stress, readiness, recovery and fitness-age scores with its own methods; they are not direct measurements of a single bodily state.
Depending on settings, a watch may sync data with a phone, cloud account or another app. Read the relevant privacy policies and sharing controls. Do not assume that all readings are uploaded or that linking an app gives a clinician access to them.
When an alert or symptom needs care
An irregular-rhythm notification means the watch detected a pattern that needs clinical interpretation. Arrange an appointment and bring the recording. A clinician may use a twelve-lead ECG or longer monitoring, since an intermittent rhythm problem may not appear during a brief test.
Symptoms count for more than any watch reading. Chest pain, breathlessness at rest, fainting, a racing or pounding heart that does not settle, or a resting heart rate that stays higher or lower than usual for days belong with a doctor regardless of what the watch says. If you have any of these, see a GP or polyclinic without delay, and for chest pain or a suspected stroke, call 995. Ask a clinician without delay about repeated abnormal readings or an irregular-rhythm alert, even without symptoms. For an isolated unexpected result, check fit and the device instructions, repeat where appropriate and seek advice if it remains abnormal or you are concerned.
Using the watch as a tracking tool
A smartwatch can help maintain an activity habit or show a pattern worth discussing. Its usefulness depends on the feature and device. Check the model’s intended use, wear it as instructed and avoid making treatment decisions from a consumer score.
Wear it snug, go by weekly averages over single readings, and treat any alert as a reason to get checked rather than as a result. If you have symptoms or a rhythm alert, see a doctor and bring the data along. Do not change a medicine or skip a screening because a watch said everything looked fine.
A practical next step
Check the instructions for your model and look for consistent patterns rather than treating every score as a verdict. Arrange a review for an irregular-rhythm alert, and seek care for symptoms regardless of the watch reading.
Sources and further reading
These references explain the guidance and research discussed above. For advice about your own health, speak to a qualified healthcare professional.
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