Health wearables: which data from your smartwatch actually matters?

The number a wearable displays depends on the sensor, the algorithm, where the device sits, movement and the person wearing it. Two brands can use the same word, “recovery” or “stress”, and calculate it in very different ways.

Before looking at your charts, ask which decision they should inform. Tracking a return to exercise after an injury, documenting palpitations and watching how regular your sleep is are three different uses, calling for different measurements and different lengths of follow-up. Without a goal, piling up data mostly feeds worry.

Clinicians reason the same way when they bring real-world data into patient follow-up: they start from the decision, not the sensor.

Not every wearable has the same purpose. The US Food and Drug Administration (FDA) describes general wellness products as those meant to maintain or encourage a healthy lifestyle, unrelated to diagnosing, treating or preventing a disease. In France, the medicines and medical devices agency, the ANSM, points out that software with a medical purpose is a medical device and must carry the CE mark.

A regulatory clearance or a CE mark applies to the specific function and use claimed, not to the whole device: a watch can offer one medical feature alongside many wellness features. The instructions for use state the intended population, contraindications and documented performance.

A recovery or stress score is often proprietary. Without a published definition or validation, it describes a trend specific to that device, not burnout or an anxiety disorder.

Heart rate and rhythm

Most watches measure pulse with an optical sensor on the wrist. A study published in npj Digital Medicine compared several consumer and research-grade devices with an electrocardiogram: error was on average 30% higher during activity than at rest, and the authors found no statistically significant difference in accuracy across skin tones. Movement is the first factor to keep in mind.

Some watches can also record an ECG-type trace. If you feel palpitations, note the time, how long they lasted and what you were doing, then export the original recording if the app allows it: a screenshot of a summarized result says far less.

Sleep

Consumer devices estimate sleep from indirect signals, movement and pulse. Compared with the laboratory reference, polysomnography, seven devices tested in a study published in Sleep detected sleep well, wakefulness less well, and their sleep stages (light, deep, REM) were inconsistent. They help you notice regularity or change; they cannot rule out a sleep disorder. Severe daytime sleepiness, or breathing pauses noticed by a partner, call for a medical visit whatever the score says.

Activity and training load

Steps, duration, intensity and session frequency describe a load. For an athlete, reading them means taking recovery, pain, history and goals into account. A model that estimates injury risk remains probabilistic, like any predictive health tool. It informs a decision without saying for certain whether to play. That is exactly the need Joshua, an amateur tennis player describes: drained before an important tournament, with no clear indicator to choose between pushing on and easing off.

A watch is not the only way to follow these parameters, either: camera-based vital signs allow a one-off measurement with no sensor on the wrist.

A value without context is hard to interpret. For the two or three metrics that answer your question, keep:

• the date and time;
• the device and the app version;
• the measurement conditions (rest, exercise, night);
• any symptoms you felt at the same time;
• unusual events: unaccustomed exercise, an illness, travel, a change of treatment;
• any confirmation by a reference device.

A trend over several days, produced by the same device in comparable conditions, is easier to read than an isolated value. But an average can also hide an important episode. If a symptom worries you, do not wait for more data before seeking care. For chest pain, fainting or difficulty breathing, call your local emergency number.

Avoid turning up with months of charts and no question. Prepare a single page instead, one your doctor can read in a few minutes.

Ask whether they would rather receive the data before or during the appointment, and in what format. A professional is not expected to analyze a stream they neither asked for nor set up: deciding what is worth tracking is done together. This page belongs naturally in your personal health record, and in how to prepare for a doctor’s appointment.

With MyTwin for patients, data from your compatible watches, rings and sensors sits alongside your reports, lab results and imaging in one place. You build this summary from a record that is already gathered, and interpretation remains with the professional who follows you.

A step count or a weight reading does not look sensitive. Yet the CNIL, France’s data protection authority, explains that data becomes health data depending on its nature and on what it is combined with: weight cross-referenced with step count and calorie intake says something about a person’s health.

Check the permissions granted to the app, who receives the data, how long it is kept, and whether you can export and delete it. A shared account or a public fitness leaderboard can reveal your habits, your routes or your health. Revoke access you no longer need, protect the paired phone, and when you change devices, check what stays in the cloud and how to close the old account.