Smartwatch Features, Rated One by One

Smartwatches now do far more than tell the time and show notifications. Many track heart rate, sleep and workouts, and some can record an electrocardiogram or call emergency services after a hard fall. But the quality of each feature varies a lot. Some are well validated, while others give rough estimates that are easy to over-trust. Going through the features one at a time shows where smartwatches genuinely help and where their limits lie.

Notifications and convenience

Verdict: useful, with a catch. Glancing at a message or call on your wrist means picking up your phone less often, and contactless payments, timers and music controls are genuinely handy. The catch is that constant buzzing can make distraction worse rather than better. Most people find a smartwatch more pleasant after turning off all but the most important notifications.

Step counting and general activity

Verdict: reasonably accurate and motivating. Step counts from wrist devices are generally close enough for tracking trends, though they can miss steps when pushing a stroller or shopping cart and may count arm movements as steps. Research has linked higher daily step counts with lower mortality. A 2022 analysis in The Lancet Public Health found that risk fell progressively with more steps, leveling off at around 6,000 to 8,000 steps a day for adults over 60 and 8,000 to 10,000 for younger adults.

Heart rate

Verdict: good at rest, less reliable during intense exercise. Optical sensors measure heart rate by detecting changes in blood flow under the skin. Studies show they are generally accurate at rest and during steady activity like walking, but less so during intense intervals, weight lifting or activities with lots of wrist movement. A chest strap remains more accurate for serious training. Watches can also flag an unusually high or low resting heart rate, which can be a prompt to see a doctor.

ECG and irregular rhythm alerts

Verdict: a genuine medical feature, within limits. Several watches, including models from Apple, Samsung and Fitbit, have features cleared by the US Food and Drug Administration to detect signs of atrial fibrillation, an irregular heart rhythm that raises the risk of stroke. The Apple Heart Study, published in the New England Journal of Medicine in 2019 with more than 400,000 participants, found that the irregular-pulse notifications were generally accurate when compared with follow-up monitoring. These features are not designed for people already diagnosed with atrial fibrillation, don't detect heart attacks, and a normal reading does not rule out heart disease.

Blood oxygen

Verdict: interesting, but not a medical tool. Some watches estimate blood oxygen saturation. Readings can be affected by fit, movement, skin temperature and other factors, and most manufacturers describe the feature as being for general wellness. Anyone worried about breathing or oxygen levels should use a proper pulse oximeter and seek medical advice.

Sleep tracking

Verdict: useful for patterns, not precise stages. Watches estimate sleep from movement and heart rate. They are reasonably good at telling sleep from wake and at tracking total sleep time and regularity, but less accurate at dividing sleep into light, deep and REM stages compared with laboratory sleep studies. Focusing too much on nightly sleep scores can make some people anxious about sleep, which doesn't help.

Calories burned

Verdict: treat with caution. A 2017 Stanford University study that tested several popular wrist devices found that heart rate measurements were fairly accurate, but none of the devices estimated energy expenditure accurately, with errors that were often large. Using a watch's calorie figures to decide how much to eat can easily lead people astray.

Safety features

Verdict: potentially lifesaving. Fall detection on some watches can call emergency services and alert contacts if the wearer doesn't respond after a hard fall, which is particularly valuable for older adults living alone. Models with cellular connectivity can make emergency calls without a phone nearby. Some also offer crash detection and emergency SOS by pressing a button.

GPS and workouts

Verdict: good for runners, cyclists and hikers. Built-in GPS records routes, distance and pace without carrying a phone. Accuracy is generally good in open areas but can drift among tall buildings or dense trees. Dedicated sports watches tend to offer longer battery life and more detailed training data than general smartwatches.

Women's health and temperature tracking

Verdict: helpful for tracking, not for contraception unless cleared for it. Several watches and rings track skin temperature overnight, which can help estimate where someone is in their menstrual cycle and spot changes over time. Some apps use this to predict periods and fertile windows. Only a small number of apps have regulatory clearance as contraceptive methods, so a standard watch's predictions should not be relied on to prevent pregnancy.

The downsides

• Battery life: many full-featured smartwatches need charging daily or every couple of days, which conflicts with sleep tracking.
• Cost and lifespan: batteries degrade, and software support eventually ends.
• Compatibility: Apple Watch works only with iPhone, and some features of other watches depend on a particular phone brand.
• Privacy: health data is sensitive. Review privacy settings, what is shared with third-party apps, and how the company stores your data.
• Health anxiety: constant metrics can make some people anxious about normal variations.

Who benefits most

Smartwatches suit people who want gentle motivation to move, runners and cyclists who want workout data without a phone, older adults who would benefit from fall detection, and people who want fewer phone interruptions. If you mainly want step counts and sleep trends, a simpler fitness band with longer battery life may be enough. Whichever you choose, treat the data as a helpful guide rather than a diagnosis, and talk to a doctor about any readings that worry you.