Your watch or ring now tracks steps, heart rate, calories, sleep stages, heart rate variability and a readiness score. Some of those numbers are genuinely useful. Some are rough estimates. And at least one is mostly guesswork. Here is what the research says about accuracy, and how to use your data without becoming a slave to it.
What wearables get right, and wrong
A 2020 systematic review of commercially available wearables found that in lab settings, several leading brands measured steps accurately. Heart rate was more variable between brands, but still reasonably good. For energy expenditure, the calories you burn, no brand was accurate.
| Metric | How much to trust it | How to use it |
|---|---|---|
| Steps | High | Track your daily average and nudge it up |
| Heart rate | Fairly good, varies by device | Keep easy sessions easy, and spot trends in resting heart rate |
| Sleep duration | Fair. Slightly underestimates total sleep | Spot patterns in how long and how regularly you sleep |
| HRV and readiness scores | Useful as a trend | Look at weekly trends alongside how you feel |
| Calories burned | Low | Treat it as a rough guess. Do not eat it back |
Steps: the most useful number
Steps are both accurate and meaningful. A meta-analysis of 15 international cohorts, covering 47,471 adults, found that taking more steps a day was linked to a progressively lower risk of death. The benefit levelled off at around 6,000-8,000 steps a day for adults aged 60 and over, and around 8,000-10,000 for younger adults.
47,471
adults in the step count meta-analysis
6-8k
steps a day where the benefit levelled off in over-60s
8-10k
steps a day where it levelled off in under-60s
Pace mattered a little too: people whose fastest 30 minutes of walking in the day was quicker had a lower risk, even after accounting for their total steps. So start by finding your current daily average, add 1,000-2,000 steps, and make some of your walking brisk.
Heart rate and keeping easy sessions easy
Wrist-based heart rate is good enough for most training decisions. Its best use is stopping you from turning every cardio session into a hard one. For easy sessions, you should be able to talk but not sing, which is how the NHS describes moderate intensity. If your heart rate keeps creeping up on easy days, slow down.
Your resting heart rate is useful too. Look at the trend over a week or more: a run of higher than usual readings can be a sign of illness, poor sleep or accumulated fatigue.
Calories burned: treat it as a guess
Because no brand measured energy expenditure accurately in the review, the "active calories" figure on your watch is best treated as a rough estimate. If it overestimates and you eat those calories back, you can quietly wipe out your calorie deficit.
Sleep tracking
A 2025 meta-analysis of 24 studies compared consumer wrist-worn sleep trackers with polysomnography, the lab gold standard. On average, the devices underestimated total sleep by about 17 minutes and overestimated time spent awake during the night. The authors concluded they are not as reliable as polysomnography, but can still help you track your general sleep patterns.
So use your tracker for the big picture: how long you sleep, how consistent your bedtime is and how late nights or alcohol affect you. Be more sceptical about the detailed breakdown of sleep stages, and do not lose sleep over a low sleep score.
HRV and readiness scores
Heart rate variability (HRV) is the variation in time between your heartbeats, and many devices turn it into a readiness or recovery score. It is a popular idea: train hard when your HRV is high and back off when it drops.
The evidence is more modest than the marketing. A meta-analysis of endurance training guided by HRV from wearables found only a small, non-significant effect on fitness and performance compared with following a set plan, although there were fewer non-responders and a medium-sized benefit for some submaximal measures.
HRV is still worth glancing at as a trend, especially alongside sleep and resting heart rate. But a single low score is not a reason to skip training. Adjust the session if you feel rough, and train as planned if you feel fine.
How to use your data without obsessing
Pick three numbers
Daily steps, sleep duration and resting heart rate cover most of what matters.Look at weekly trends
Single days are noisy. Compare this week with last week.Change one thing at a time
Add steps, move your bedtime earlier or take a lighter training week, then see what happens.Check in with how you feel
Your data is one input. Your energy, mood and performance in the gym are just as important.
BarbellBites can sync your steps automatically from Apple Health, so they count towards your goals, streaks and step challenges with friends, alongside your workouts and food.
FAQs
Which fitness tracker is the most accurate?
Are smartwatch calorie counts accurate?
How many steps a day should I aim for?
Is HRV worth tracking?
Sources
- 1.Fuller D, et al. (2020). Reliability and validity of commercially available wearable devices for measuring steps, energy expenditure, and heart rate: systematic review. JMIR mHealth and uHealth.
- 2.Paluch AE, et al. (2022). Daily steps and all-cause mortality: a meta-analysis of 15 international cohorts. The Lancet Public Health.
- 3.Lee YJ, et al. (2025). Performance of consumer wrist-worn sleep tracking devices compared to polysomnography: a meta-analysis. Journal of Clinical Sleep Medicine.
- 4.Düking P, et al. (2021). Monitoring and adapting endurance training on the basis of heart rate variability monitored by wearable technologies: a systematic review with meta-analysis. Journal of Science and Medicine in Sport.
- 5.NHS. Walking for health.