The short answer: if you track anything at home, make it blood pressure, cardiorespiratory fitness, daily activity, waist size relative to height and sleep regularity. These have strong links to long-term health in large studies, and you can do something about each of them. Resting heart rate earns a place as a cheap trend. Most of the rest, from body fat percentages and HRV scores to glucose graphs for people without diabetes, is interesting but rarely changes what you should do.
That is a narrower list than the wearables industry would like. Here is how we reached it, metric by metric, with the studies behind each one.
How we judged each health metric
Martin Eriksen worked in the Danish pharmaceutical industry, where a biomarker has to earn its keep: it must be measurable reliably, it must be linked to outcomes that matter (illness, disability, death), and changing it should lead to a decision. We applied the same three questions to the numbers on your wrist, your scale and your phone.
| Metric | Link to long-term health | Home measurement quality | Changes what you do? | Verdict |
|---|---|---|---|---|
| Blood pressure | Very strong | Good with a validated upper-arm cuff | Yes | Track |
| Cardiorespiratory fitness (VO2 max) | Very strong | Fair (wearable estimates) | Yes | Track as a trend |
| Daily steps and active minutes | Strong | Good | Yes | Track |
| Waist-to-height ratio | Strong | Good (tape measure) | Yes | Track monthly |
| Sleep duration and regularity | Moderate to strong | Fair | Yes | Track |
| Resting heart rate | Moderate | Good | Sometimes | Track as a trend |
| Weight | Moderate | Very good | Sometimes | Track as a weekly average |
| Heart rhythm (AFib alerts) | Strong for those affected | Good at flagging, not diagnosing | Yes, see a GP | Useful over 65 or at risk |
| HRV | Weak for individuals | Variable | Rarely | Optional |
| Body fat % (smart scale) | Moderate | Poor | Rarely | Ignore day to day |
| Sleep stages | Weak | Poor to fair | Rarely | Ignore |
| Glucose (no diabetes) | Unclear | Good sensors, unclear meaning | Rarely | Skip unless advised |
The health numbers worth tracking
Blood pressure: the most valuable number you can measure at home
High blood pressure is a leading driver of stroke, heart attack, kidney disease and dementia, and it usually causes no symptoms. That combination makes it the single best metric to measure at home if you are over 40 or have risk factors such as diabetes, kidney disease or a family history.
NICE guidance (NG136) uses a home average of 135/85 mmHg or higher to confirm stage 1 hypertension. Use a validated upper-arm monitor, measured twice morning and evening over four to seven days. In England, adults aged 40 to 74 without existing conditions are invited for an NHS Health Check every five years, which includes blood pressure, and many pharmacies offer free checks to over-40s. Our guide to the best smart blood pressure monitors lists validated models and the full measuring routine.
How often: a week of readings once or twice a year if normal; more often if your GP asks.
Cardiorespiratory fitness: the metric with the biggest effect size
Fitness, usually expressed as VO2 max, is one of the strongest predictors of how long people live. A meta-analysis of 33 studies in JAMA (Kodama et al., 2009) found that each one-MET increase in fitness was associated with a 13 per cent lower risk of death from any cause (JAMA, 2009). A study of more than 120,000 adults who had treadmill tests at the Cleveland Clinic (Mandsager et al., JAMA Network Open, 2018) found the fittest group had the lowest mortality, with no upper limit to the benefit (JAMA Network Open, 2018).
Watches from Garmin, Apple and others estimate VO2 max from heart rate and pace. The absolute figure can be several points out, but the trend is useful. If it is rising, your training is working.
How often: check monthly, not daily.
Daily activity: steps and active minutes
Steps are crude but robust. A meta-analysis of 15 cohorts and 47,471 adults in The Lancet Public Health (Paluch et al., 2022) found that the risk of death fell as steps increased, levelling off at about 6,000 to 8,000 steps a day for adults over 60 and 8,000 to 10,000 for younger adults (Lancet Public Health, 2022).
The UK Chief Medical Officers’ guidelines ask adults for at least 150 minutes of moderate activity, or 75 minutes of vigorous activity, a week, plus muscle-strengthening work on two days. Most trackers record active minutes; few record strength work automatically, so log it yourself.
How often: a weekly total matters more than any single day.
Waist-to-height ratio: the tape measure beats the scale
NICE advises that people keep their waist measurement to less than half their height, because fat around the middle is linked to type 2 diabetes and heart disease more closely than weight alone. It needs no gadget: measure halfway between your lowest rib and the top of your hip bone, breathing out naturally.
How often: monthly.
Sleep: duration and regularity, not stages
The NHS suggests most adults need seven to nine hours. Increasingly, research suggests regularity matters as well: going to bed and waking at consistent times. A UK Biobank analysis of 60,977 people wearing wrist accelerometers (Windred et al., Sleep, 2024) found sleep regularity was a stronger predictor of mortality than sleep duration, with the most regular sleepers at 20 to 48 per cent lower risk of death than the least regular (Sleep, 2024).
Wearables are reasonable at telling sleep from wake and poor at classifying stages. A comparison of seven consumer devices against polysomnography (Chinoy et al., Sleep, 2021) found most performed as well as or better than research actigraphy for sleep and wake, with inconsistent results for sleep stages (Sleep, 2021). Use your tracker for bedtimes, wake times and total sleep. Our piece on sleep technology that does more than track covers what helps.
Useful trends, not targets
Resting heart rate
A lower resting heart rate generally reflects better fitness. In the Copenhagen Male Study (Jensen et al., Heart, 2013), higher resting heart rate was associated with higher mortality even after accounting for fitness (Heart, 2013). Wearables measure it well overnight. A sustained rise of several beats over your usual level can signal illness, overtraining, poor sleep or alcohol. A resting rate persistently above 100, or below 50 with dizziness or fainting, is worth discussing with your GP.
Weight
Weight is easy to measure accurately, but it bounces around by a kilo or more day to day with fluid and food. Use a weekly average. Unexplained weight loss, especially more than 5 per cent over a few months, is a reason to see your GP.
Heart rhythm alerts
Atrial fibrillation raises stroke risk and often has no symptoms. In the Apple Heart Study of 419,297 participants (Perez et al., New England Journal of Medicine, 2019), 0.52 per cent received an irregular pulse notification; among those whose follow-up ECG patch could be analysed, 34 per cent had atrial fibrillation confirmed (NEJM, 2019). An alert is a reason to see your GP, not a diagnosis. These features are most useful for people over 65 or with risk factors such as high blood pressure.
Metrics you can safely ignore day to day
Heart rate variability
HRV, the variation in time between heartbeats, reflects how your autonomic nervous system is balancing stress and recovery (Shaffer and Ginsberg, Frontiers in Public Health, 2017). Low HRV is associated with poorer health at population level, but individual numbers vary hugely with age, genetics, alcohol, illness and measurement method. Compare yourself only with yourself, over weeks. Our explainer, what is heart rate variability and why does it matter, goes deeper, and our WHOOP vs Oura vs Garmin comparison looks at how each device uses it.
Body fat percentage from smart scales
Bioimpedance scales are accurate for weight but not for fat. A 2021 study in JMIR mHealth and uHealth (Frija-Masson et al.) found three smart scales underestimated fat mass against DXA by a median of 2.2 to 4.4 kg (JMIR mHealth, 2021). Waist-to-height ratio tells you more for free. See our guide to the best smart scales for how to read them.
Blood oxygen (SpO2) on a watch
Wrist SpO2 sensors are not medical pulse oximeters, and readings can be less accurate in people with darker skin, cold hands or poor circulation. Healthy people at sea level rarely need to track SpO2. If you have lung or heart disease, use the device your clinical team recommends.
Glucose for people without diabetes
Over-the-counter glucose sensors are now sold to people without diabetes. In healthy adults, glucose stays in a narrow range most of the time: a study of 153 non-diabetic participants aged 7 to 80 (Shah et al., Journal of Clinical Endocrinology and Metabolism, 2019) found they spent a median 96 per cent of the day between 70 and 140 mg/dL (JCEM, 2019). Evidence that tracking post-meal spikes improves health in people without diabetes is thin. Our Stelo vs Lingo comparison sets out who these devices are and are not for.
Readiness, recovery and “body age” scores
These are proprietary blends of HRV, resting heart rate, sleep and activity. They can be motivating, but the formulas are unpublished and not validated against health outcomes. Use them to prompt reflection, not to override how you feel.
When to stop tracking and see a professional
Self-tracking should never delay care. See your GP or pharmacist if:
- Your home blood pressure average is 135/85 mmHg or higher.
- You get an irregular rhythm alert, or have palpitations, dizziness or fainting.
- You lose weight without trying.
- You are thirsty all the time, passing urine more often or feeling unusually tired, which can be signs of diabetes.
Call 999 for chest pain, sudden breathlessness, signs of stroke (face drooping, arm weakness, speech problems) or a very high blood pressure reading with symptoms.
If tracking makes you anxious, or you find yourself checking numbers compulsively, it is fine to stop. Health is the goal, not the data.
Health metrics FAQs
What is the single most important health metric to track?
For most adults over 40, blood pressure. It is common, usually symptomless, strongly linked to stroke and heart disease, and treatable. A validated upper-arm monitor is inexpensive.
Is resting heart rate a good indicator of health?
It is a useful trend. A lower resting rate usually reflects better fitness, and a sustained rise can signal illness or overtraining. Compare it with your own baseline rather than with other people.
How many steps a day should I aim for?
Research suggests benefits level off at about 6,000 to 8,000 steps for adults over 60 and 8,000 to 10,000 for younger adults. Any increase from a low base helps.
Is VO2 max from a smartwatch accurate?
It is an estimate that can be several points out compared with a lab test. Watch the trend over months rather than the number on any one day.
Should I track HRV?
Only if it helps you notice patterns such as the effect of alcohol or poor sleep. Individual HRV varies too much to compare with others, and day-to-day swings are normal.
Do I need a glucose monitor if I don’t have diabetes?
Probably not. Evidence of benefit in people without diabetes is limited. If you have symptoms or risk factors for diabetes, ask your GP about an HbA1c blood test instead.
