Millions of people now wear smartwatches, rings and chest straps. They track heart rate, heart rate variability (HRV), steps, sleep and, increasingly, skin temperature and blood oxygen. Continuous glucose sensors have joined them, bought even by people without diabetes. These devices can be a useful mirror of your lifestyle. Their numbers, however, are not lab results, and not all of them are equally accurate.
How to read the numbers: Fact = a finding from research or biology
for adults by expert bodies
adult resting heart rate range
of one sleep cycle
How the devices measure
Most watches and rings use photoplethysmography (PPG): green or infrared light shines into the skin and a sensor tracks tiny changes in blood flow with each heartbeat. Chest straps, by contrast, record the heart’s electrical activity, much like an ECG. That makes them more accurate during exercise: optical wrist sensors struggle mainly with sudden changes in intensity, arm movement, a loose fit or cold.
Resting heart rate and HRV: watch the trends
Resting heart rate measured overnight is one of the most reliable figures wearables provide. A long-term decline often accompanies improved fitness, while a sudden rise of several beats can signal lack of sleep, alcohol, overheating, stress or an incoming infection.
HRV describes the variation in time between heartbeats and mainly reflects the activity of the parasympathetic nervous system. Values are highly individual and depend on age, genetics and the measurement method. Comparing yourself with other people therefore makes no sense. What is useful is tracking your own average over several weeks and noticing marked deviations. Different manufacturers also calculate HRV differently, so figures from two devices may not be comparable.
Sleep: accuracy versus polysomnography
The gold standard for measuring sleep is polysomnography in a sleep lab, which records brain waves, eye movements and muscle tone. Wearables do not measure brain activity. They estimate sleep stages from movement, heart rate and HRV using algorithms. They are fairly good at telling sleep from wakefulness and estimating total sleep time. They are worse at catching brief awakenings, and agreement on individual stages (light, deep and REM sleep) is only moderate. A reading of “deep sleep: 48 minutes” is an estimate, not a diagnosis.
One night with a “low score” means little. What matters are repeated changes over weeks, especially when they match how you feel.
Glucose sensors for people without diabetes
A continuous glucose monitor (CGM) measures sugar in the interstitial fluid under the skin, and its readings lag behind blood levels. For people with diabetes it is a proven treatment tool. For healthy people, wearing one for a short period can serve as a learning tool: it shows how you respond to particular meals, the order in which you eat foods, a walk after eating or a short night.
The risk lies in over-interpreting the results. Brief rises in glucose after a meal are normal in a healthy person and are not evidence of disease. Sensor accuracy is also lower at low glucose levels and when the sensor is pressed on during sleep. There is no evidence that glucose monitoring in healthy people improves health on its own. Avoiding carbohydrates excessively because of the curve can lead to an unnecessarily one-sided diet.
- Strong evidence Resting heart rate measured by optical sensors is fairly accurate; chest straps measure heart rate during exercise more accurately than the wrist.
- Moderate evidence Devices estimate total sleep time well, but sleep stages and night-time awakenings only approximately.
- Weak evidence The benefit of CGM for healthy people – useful for learning, but evidence of better health is missing.
- No evidence Composite “readiness scores” or “biological age” from apps as a reliable measure of health.
Orthosomnia: when data make sleep worse
Clinicians describe a phenomenon called orthosomnia – an anxious pursuit of a “perfect” sleep score. People then spend more time in bed than they need or worry about every poorer night. Paradoxically, this makes their sleep worse. If the numbers stress you more than they help, try putting the device away for a few weeks or switching off the daily score.
Data privacy
Data on your heart rate, sleep, menstrual cycle or location are among the most sensitive information about you. In the European Union, health data are a special category of personal data, but it is still worth reading the terms. Find out where the data are stored, whether the manufacturer shares them with third parties and whether you can export and permanently delete them. Turn off features you do not need and be careful when linking apps.
A wearable is a tool for self-knowledge, not a doctor. If it alerts you to an irregular heart rhythm or a persistently raised heart rate, have it checked by a doctor – and if it reassures you, do not rely on it for symptoms that need examination.
How to use the data sensibly
- Track a few key indicators – for example resting heart rate, HRV and sleep regularity – and ignore the rest.
- Measure the same way every time: same device, same wrist, measured overnight or right after waking.
- Look at weekly averages, not individual days.
- Compare the data with how you feel. If you feel great and the score is low, trust your body.
- With a CGM, set a clear goal and a limited time – for example two weeks to test your usual meals.
- When adjusting training or routines, make only one change at a time so you know what worked.
Who should be cautious
- People prone to anxiety or insomnia – constant monitoring can make these problems worse.
- People with eating disorders, now or in the past – tracking calories and glucose can be a trigger.
- People with heart disease or diabetes – a consumer device does not replace a medical device; do not change your treatment based on its readings, talk to your doctor.
- People with an implanted pacemaker or defibrillator – check compatibility for devices with electrical sensors.
- People with sensitive skin – if the skin under a sensor or patch becomes irritated, stop using it.
Conclusion
Wearables can reveal long-term patterns you would otherwise miss – the effect of late meals, alcohol, stress or training on your heart rate and sleep. You can trust them most for resting heart rate and total sleep time, and less for sleep stages and composite scores. Use them as a compass, not a judge: follow trends, make small changes and leave the final word to your body and your doctor.