Modern smartwatches measure your pulse, estimate your sleep and even check your blood oxygen. The data can be genuinely useful, as long as you know which numbers to trust.
Heart rate: good for trends
Watches use optical sensors that shine light into your skin and measure changes in blood flow. Resting heart rate and steady workouts are usually tracked well. Accuracy drops during intense interval training or when the watch is loose.
ECG: a real medical feature on some models
Some watches can record a single-lead ECG and flag possible atrial fibrillation. These features went through regulatory clearance in some countries, but they still do not replace a doctor’s full ECG.
Blood oxygen: a rough guide
Wrist-based SpO2 readings can be affected by movement, fit and skin temperature. Treat a single low reading with caution, and look at trends instead.
Sleep: estimates, not measurements
Watches estimate sleep stages from movement and heart rate. Total sleep time is usually reasonable, but the split between light, deep and REM sleep is an educated guess.
Calories: the least reliable number
Calorie burn is calculated from heart rate, movement and your profile, and can be off by a wide margin. Use it to compare days, not to plan exact meals.
Steps: surprisingly good
Step counting is one of the more dependable numbers on a smartwatch, especially for normal walking. It can miss steps when you push a stroller or shopping cart, since your wrist barely moves, and it can add a few when you wave your arms while talking. Over a week, those errors mostly even out, which makes step trends a useful measure of how active you are.
Heart rate variability and readiness scores
Many watches now report heart rate variability (HRV), the small changes in time between heartbeats, usually measured overnight. HRV tends to drop when you are stressed, sick, short on sleep or recovering from hard training. The number itself varies hugely between people, so comparing yours with a friend’s tells you nothing. Comparing it with your own normal range can be informative.
Readiness, recovery and body battery scores combine HRV, resting heart rate and sleep into one figure. They are a company’s own formula, not a medical measurement. Treat them as a nudge to check in with how you feel, not an instruction to skip a workout.
Alerts worth taking seriously
Some notifications deserve attention even though the watch is not a medical device in the full sense:
- Irregular rhythm notifications that suggest possible atrial fibrillation, especially if they repeat.
- High or low heart rate alerts while you are resting.
- Fall detection, which can call emergency services if you don’t respond. It is worth turning on for older relatives.
None of these is a diagnosis, but each is a reason to talk to a doctor, and to bring the watch data along.
How to use the data
- Focus on long-term trends rather than single readings.
- Wear the watch snugly, a finger’s width above the wrist bone.
- Keep your age, height and weight up to date in the health app, since many estimates rely on them.
- Don’t let the numbers make you anxious. If checking sleep scores keeps you awake, turn them off for a while.
- Talk to a doctor if alerts or symptoms worry you. A smartwatch is a helpful signal, not a diagnosis.
What we think
Smartwatches are best at showing trends in your own body over weeks and months: whether your resting heart rate is creeping up, whether you are sleeping less or moving more. They are weakest at single precise measurements, and they are not medical devices in the full sense.
Use the data as a conversation starter, not a diagnosis. If an alert worries you, or a trend changes without explanation, take the watch data to your doctor rather than searching for answers online.