Understanding your data

How do you make sense of your tracker's data?

Treat tracker numbers as rough guides, not exact measurements. The data isn't standardised and the formulas behind it are usually hidden¹. Step counts can differ by 20% or more between devices, and some devices may miss steps at very slow walking speeds⁶. A tracker also can't diagnose anything, so if a number worries you, or you have symptoms, talk to a doctor rather than relying on the device¹.

Patterns are more useful than single readings. Line charts show trends and changes over time, while bar charts suit reading a single day⁷.

• Use the trend view. Look at the line over time rather than worrying about one day's number.
• Take resting heart rate in the morning, before you get out of bed⁸. Compare it with your own earlier readings, not other people's.
• Read total sleep time with sleep efficiency (the share of your time in bed that you're asleep). A longer night with lower efficiency may simply mean more restlessness⁹.
• Pick goals that suit where you are now. The popular 10,000-steps-a-day target isn't shown to be necessary⁶, and amounts of activity smaller than the guideline targets still bring benefit¹⁰. Keep goals gentle.

What can heart rate tell you during exercise?

Heart rate is a rough guide to how hard your body is working. A common target for moderate effort is 50 to 70 per cent of your maximum heart rate, with maximum estimated as 220 minus your age. Because that is only an estimate, use it with caution, and if you're just starting regular exercise, stay at the lower end and build up gradually².

You can also judge effort without a device. With the talk test, if you can talk comfortably but not sing, you're at a moderate effort². Or use a self-rated effort scale: a rating of 11–13, described as "low", is suggested for less trained people¹¹.

How far to trust your watch: wrist heart rate devices are generally acceptable for most common activities, and sit close to a reference measure at rest, in sleep and from walking to running³. They read lower during resistance training and cycling, so don't rely on a wrist reading alone there³. Movement, skin pigmentation, tissue thickness, ambient light and temperature can also disturb readings⁴. Hot weather, caffeine, time of day, hormone changes, stress or anxiety and smoking can raise heart rate too, not just exercise².

If you take medicines that can affect your heart rate, readings may not reflect how hard you're working. Ask your doctor or pharmacist how best to judge effort.

Don't push on to hit a heart rate number if you feel sick, sweaty or unwell, or very breathless or dizzy. If it doesn't settle, use the emergency advice at the end of this page. If it has settled, see a doctor soon.

What can modern wearables measure besides steps?

Beyond steps, wearables can track heart rate and sleep, and some can look for heart rhythm problems. Small, low-cost light-based sensors in wrist-worn wearables have made it easier to collect heart rate in everyday life³. Many devices also store things you enter yourself, such as food eaten, physical activities or weight⁷. Pedometers and fitness trackers may help you count steps, calories and minutes of physical activity¹².

Not every measure is equally dependable. Wrist heart rate is the most reliable of these³. Sleep stage estimates are the weakest, and all devices still have room to improve at identifying them⁴. Wearables also vary in how precisely they detect common heart rhythm problems¹³. Treat any blood oxygen or calorie figure as a rough guide only.

If a heart rhythm alert comes with chest pain, breathlessness or feeling faint, use the emergency advice at the end of this page. If you're worried about your heartbeat otherwise, have it checked by a doctor rather than relying on the device alone.

Start with the simplest measures: steps, minutes of activity and general heart rate trends. Leave sleep stages until later.

How accurate is sleep tracking on a wearable?

Wearables are fairly good at telling that you were asleep, but weaker at spotting when you were awake, and weakest at sleep stages⁴.

Overnight sleep lab testing (polysomnography) is the gold-standard benchmark⁴. Compared with it, wrist trackers are less reliable for total sleep time, sleep efficiency and sleep latency (how long it takes you to fall asleep), though they can still show general patterns if read carefully¹⁴. Movement sensors may overestimate sleep by missing quiet, still wakefulness, such as lying down reading or watching television⁴. A sensor that measures only limb movement uses activity as a stand-in for being awake and stillness as a stand-in for sleep, so it has limited ability to tell sleep stages apart¹⁵. Most devices also do worse on restless nights or when it takes longer to fall asleep¹⁶.

What this means for you:

• Look at patterns across many nights rather than one night's score.
• Treat the stage breakdown (light, deep, REM) as a rough estimate, not a measurement.
• If you have ongoing trouble sleeping, see a doctor or other suitable practitioner rather than relying on the numbers. Your tracker log may help the conversation.

Do sleep apps help you sleep better?

Structured sleep-app programmes can help people with insomnia or sleep disturbances, but simple sleep tracking is not the same thing. Structured programmes improved self-reported insomnia and sleep disturbances, and also led to fewer night-time awakenings, a little more total sleep and quicker falling asleep. Daytime sleepiness and sleep efficiency did not improve⁵.

The benefit may be smaller in real life than it looks. Improvement is often judged by statistical rather than clinically meaningful change, and objectively measured sleep results are rarely reported⁹. Sleep-medicine guidance says many sleep apps make claims without empirical evidence or national guidelines behind them, and that they are not substitutes for treatments that have been rigorously tested¹. They may still raise awareness of sleep and encourage people to seek help¹.

What to do:

• Try a regular sleep routine, and keep it the same even on days off, as this can make it easier to fall asleep¹⁷.
• If you try an app, choose one that says what evidence or guidelines it is based on. Many don't.
• If you only track your sleep, use it to spot patterns and notice habits, not to score yourself.
• Don't use an app in place of treatment or advice you've been given, and see a doctor if sleep problems continue.

When to check with a health professional

If you're starting exercise after a long break, it's always a good idea to see your doctor first.

For an injury or a health condition, see a health professional, such as your doctor or a physio. If you live with a long-term condition, talk to your doctor or other health professional about goals that suit you, as tracker readings may not reflect how well your body is doing.

Call your local emergency number for chest pain or pressure (or pain spreading to the arm, neck, jaw or back, especially with sweating or feeling sick), severe breathlessness, fainting or collapse, signs of a stroke, a serious injury, or someone who isn't safe.

For a sore muscle or joint, ease off, and see a doctor or other health professional if it persists or gets worse. Feeling sick, sweaty or unwell, unexplained discomfort, or breathlessness or dizziness that doesn't settle isn't something to ease off for. Use the emergency advice above, or if it has settled, see a doctor soon.