Health Wearables, Metric by Metric
Smartwatches and fitness rings now estimate heart rhythm, blood oxygen, sleep stages, skin temperature and more. Some of these measurements are accurate enough to be cleared as medical devices; others are rough estimates. Knowing which is which helps you decide how much weight to give the numbers.
Heart rate
Reliability: good at rest, weaker during intense exercise. Optical sensors shine light into the skin and detect changes in blood volume. They're generally accurate when you're still, but readings can lag or jump during interval training or weightlifting, when wrist movement interferes. A chest strap is more accurate for training.
ECG and irregular rhythm alerts
Reliability: good for one specific job. Several watches, including Apple Watch, Samsung Galaxy Watch and Fitbit models, have FDA-cleared features that check for signs of atrial fibrillation, an irregular rhythm that raises stroke risk. In the Apple Heart Study, published in the New England Journal of Medicine in 2019, about 0.5 percent of more than 400,000 participants received an irregular rhythm alert. Among those who then wore a medical ECG patch, about a third were found to have atrial fibrillation. An alert is a reason to see a doctor, not a diagnosis, and a clean reading doesn't rule out heart problems.
Blood oxygen (SpO2)
Reliability: rough. Most wearables that show blood oxygen are sold for general wellness, not medical use. Readings are affected by fit, movement, cold fingers and nail polish. Research on fingertip pulse oximeters has also shown that they can overestimate oxygen levels in people with darker skin, and the FDA has issued guidance on that problem. Treat a single low reading with caution, and don't rely on a wearable to monitor a lung or heart condition.
Sleep stages
Reliability: decent for total sleep, weak for stages. Wearables estimate sleep from movement and heart rate. They're reasonably good at telling how long you slept, but they often misclassify light, deep and REM sleep compared with an overnight sleep lab study. Trends over weeks are more useful than any one night's breakdown.
Heart rate variability and readiness scores
Reliability: useful as a personal trend. Heart rate variability (HRV), the small variation in time between heartbeats, tends to drop with illness, stress, poor sleep and hard training. Comparing your own numbers over time is meaningful; comparing them with other people's is not, because normal ranges vary widely.
Skin temperature
Reliability: good for change, not absolute temperature. Rings and watches track small shifts in temperature overnight. These are used in some cycle-tracking features and can hint at an oncoming illness, but they don't replace a thermometer.
Women's health features
Reliability: varies by feature. Several watches and rings estimate ovulation and predict periods using temperature and heart rate patterns. These estimates can help with cycle awareness, but they are not reliable contraception unless the specific app has been cleared for that purpose, and even then it depends on careful daily use. A few apps have received FDA clearance as digital contraceptives; most cycle trackers have not.
Calories and steps
Reliability: steps good, calories poor. Step counts are generally close enough for tracking daily activity. Calorie estimates are much less reliable. A 2017 Stanford University study tested seven popular devices and found that while most measured heart rate reasonably well, none estimated energy expenditure within 20 percent of a lab measurement, and the best was off by about 27 percent. Using "calories burned" figures to decide how much to eat can easily lead you astray.
Blood pressure
Reliability: limited and feature-specific. A few devices offer blood pressure features. Some require regular calibration against a standard cuff and are only approved in certain countries. In 2025 Apple received FDA clearance for a feature that looks for patterns over about 30 days that may suggest high blood pressure and then prompts the user to check with a cuff; it does not display blood pressure readings. A validated upper-arm cuff remains the standard for home monitoring.
Fall detection
Reliability: helpful but imperfect. Watches with fall detection can call emergency services if the wearer doesn't respond after a hard fall. They can miss slow falls and occasionally trigger on vigorous activity. For older adults living alone, the feature can be a useful backup to other safety measures.
Using the data well
- Look at trends over days and weeks rather than single readings.
- Wear the device snugly and in the same position each day.
- Share consistent, unusual patterns with a doctor, along with how you feel.
- Seek care for symptoms such as chest pain, fainting or shortness of breath, whatever your watch says.