This article offers general information about consumer devices. It is not medical advice. If you have a health concern, speak to a qualified professional rather than relying on a wearable.
What these devices measure
A smartwatch or fitness tracker is a small computer with sensors, worn on the wrist or sometimes on a finger or arm. Different models include different features, but common ones are below.
Movement. An accelerometer, and often a gyroscope, detects motion. Software counts steps, recognises some activities and estimates distance and calories. This is a calculation based on movement and personal details such as age, height and weight, not a direct reading of energy use.
Heart rate. Most devices use optical sensing, called photoplethysmography. Green or other lights shine into the skin, and a sensor detects how much light is absorbed or reflected as blood volume changes with each heartbeat.
Location. Many have GPS or use the connected phone to map runs, walks and rides.
Sleep. Movement and heart signals are used to estimate when you were asleep and, in some cases, which sleep stage you were in.
Other features. Some models include an electrical heart sensor that can record a brief rhythm trace, a skin temperature sensor, a blood oxygen estimate, fall detection, and an altimeter. Availability varies by product and region, and not every feature is cleared for medical use everywhere.
How accurate are the readings
Accuracy varies by measurement, and no single answer applies to every device.
- Steps are usually reasonable for walking, though they can be off when arms are still, such as while pushing a stroller, or when arms move without walking.
- Heart rate at rest is often close to a reference measurement. During vigorous or irregular exercise, readings can lag or drift, particularly if the strap is loose or the wearer is moving a lot. Chest straps, which pick up electrical signals, are generally regarded as more dependable for workouts.
- Calories burned is a rough estimate. Treat it as a ballpark, not a precise figure.
- Sleep stages are inferred, not directly observed. Wrist devices tend to be better at telling sleep from wakefulness than at identifying specific stages.
- Blood oxygen readings from wrist devices can be affected by skin tone, fit, movement and cold, and are best seen as a rough indicator.
- Skin and sensor conditions. Tattoos, sweat, hair, cold weather and a loose fit can all disturb optical sensors.
Independent research has generally found that consumer devices can be useful for tracking trends but vary in their precision. Trends over time, such as whether your resting heart rate is gradually changing, are often more meaningful than a single reading.
Features that flag an irregular heartbeat can prompt people to seek a check, but they are screening aids, not diagnoses. They can produce false alarms, and they can miss problems. A normal reading from a watch does not mean that all is well.
Potential benefits
- Motivation. Seeing activity goals and streaks can encourage some people to move more. Others find them stressful, or stop caring once novelty fades.
- Awareness. Information about sleep, activity and heart rate can help people notice patterns.
- Convenience. Notifications, contactless payments via mobile wallets, timers and navigation are available without taking out a phone.
- Safety features. Fall detection and emergency calling can help in some situations, though they can also trigger by mistake.
- Training. Runners and cyclists can use pace, distance and heart-rate zones to structure sessions.
Drawbacks
- Cost and upkeep. Devices need charging, often daily or every few days, and may need replacing as batteries age.
- Over-reliance. Focusing on numbers can lead to anxiety or to ignoring how you actually feel.
- Inaccuracy. Mistaken readings can lead to needless worry or false reassurance.
- Distraction. Notifications can interrupt rather than help.
- Skin irritation. Some people react to straps or materials, or to wearing a watch while sleeping.
- Limited support. Like other connected gadgets, they may stop receiving updates after a few years, which makes device support lifespan worth checking before buying.
Data privacy
Wearables collect some of the most personal data around: movements, heart rate, sleep, location and sometimes health records. This data typically syncs to a phone app and a company’s servers.
Things worth checking include:
- What the company says it collects, and whether it is shared with partners or used for advertising. The privacy policy is the place to look, even if it is long.
- Whether you can delete your data and your account.
- Which app permissions the companion app asks for, since some permissions go beyond what the device needs.
- Whether data is shared with third-party apps, employers or insurers through wellness programs, and whether that is optional.
- How the company has handled security, and whether it has had a data breach.
Health data is covered by specific legal protections in some places, but not in all, and a consumer device company is not always covered by the rules that apply to doctors and hospitals. Rules differ by country.
Choosing sensibly
Decide what you actually want: step counting, workout tracking, sleep trends, notifications or a combination. A simpler band can suffice if you just want steps and heart rate. Check phone compatibility, battery life, how long the maker promises updates, and which data stays on the device.
A wearable can be a useful prompt to move and rest, but it works best as one input among several, not as a judge of your health.