Your smartwatch knows how long you slept, how many steps you took, your resting heart rate, heart rate variability, blood oxygen level, body temperature, stress score, calories burned, estimated VO2max, body composition, and another dozen numbers that change every hour.
It’s tempting to look at all of this and feel like you’re in control of your health. But the question few people ask is: of those 20+ metrics, how many actually change what you do on a daily basis?
The honest answer is that most of the numbers your watch shows are, at best, curiosities. At worst, sources of anxiety. Let’s separate what’s worth tracking from what’s pure vanity.
What’s worth tracking (evidence-backed)
These are metrics with solid scientific backing that can inform real decisions about your health when tracked as trends over time.
Sleep duration and quality
Sleep is probably the most impactful metric a wearable can track. Decades of research connect sleep quality to virtually everything: immunity, metabolism, cardiovascular health, cognition, mood, athletic performance.
Most wearables can measure total duration and sleep stages with reasonable accuracy. Don’t expect the same precision as a clinical polysomnography study, but the trend over weeks is reliable enough to identify patterns — and act on them.
How to use it: Look at your weekly average, not any single night. If your average is dropping, investigate what changed in your routine.
Resting heart rate (RHR)
RHR is a simple, well-validated indicator of cardiovascular health. A lower RHR generally indicates better fitness. More important than the absolute number is the trend: if your RHR is progressively rising without an obvious reason, it could signal accumulated stress, poor recovery, or the onset of illness.
How to use it: Track the weekly average. Gradual decreases indicate improving fitness. Sustained increases deserve attention.
Heart rate variability (HRV)
HRV measures the variations between heartbeats and is considered one of the best indicators of autonomic nervous system balance. A higher HRV generally indicates better recovery capacity and lower physiological stress.
The important detail: HRV is extremely individual. Your number isn’t comparable to anyone else’s. What matters is your own trend over weeks and months.
How to use it: Watch the weekly trend. If your HRV is consistently lower than your personal baseline, it might be time to reduce training intensity, improve sleep, or manage stress.
Steps and daily movement
This might seem too basic for this list, but step counting remains one of the metrics most strongly correlated with health outcomes in large population studies. Research involving hundreds of thousands of participants shows that more daily movement is associated with lower all-cause mortality — with significant benefits starting around 7,000-8,000 steps per day.
How to use it: Don’t fixate on “10,000 steps” — that number is marketing, not science. Find a target that works for your lifestyle and stay consistent.
Active minutes and exercise tracking
The World Health Organization recommends at least 150 minutes of moderate activity or 75 minutes of vigorous activity per week. Wearables do a good job tracking whether you’re meeting those targets.
How to use it: Track moderate-to-vigorous activity minutes per week. It’s more useful than calories burned and more aligned with what research actually measures.
Potentially useful metrics (with caveats)
These metrics have real applications, but with important limitations that most manufacturers don’t highlight.
Blood oxygen (SpO2)
SpO2 can be useful for sleep apnea screening — if your levels drop significantly during the night, it may be worth investigating with a healthcare professional. In the day-to-day life of a healthy person at sea level, the variation is minimal and rarely actionable.
Stress score
Most wearables calculate a “stress score” derived from HRV and other signals. It’s a useful metric as a trend — if your score has been consistently high for weeks, something deserves attention. But as a daily isolated number, it fluctuates too much to be meaningful.
Body temperature trends
Variations in basal temperature can indicate the onset of illness before symptoms appear. For people who menstruate, temperature tracking is useful for monitoring the menstrual cycle. Outside these contexts, daily variations are difficult to interpret.
ECG (electrocardiogram)
Some wearables offer single-lead ECG that can detect atrial fibrillation. It’s a legitimate screening tool — but it’s only useful if you know what to do with the result. An abnormal ECG needs medical evaluation; a normal ECG doesn’t rule out other cardiac problems.
The vanity metrics (look good, do little)
This is where most of the shiny numbers live. Metrics that make you feel like you’re monitoring your health but rarely change what you actually do.
Calories burned
This is perhaps the most misleading wearable metric. Research shows that wrist-based sensors have 30% to 90% error rates in estimating caloric expenditure. That means your “500 calories burned during a run” could have been 250 or 950. Making food decisions based on this number — like “I burned 400 calories, I can have an extra burger” — is a gamble, not science.
VO2max estimate
Real VO2max is measured in a lab, with a gas analysis mask, during an exercise protocol pushed to exhaustion. What your watch shows is a rough estimate based on heart rate and walking or running speed. It might give a general sense of trend, but it’s not comparable to an actual test and shouldn’t be used as a performance metric.
Body composition (from the wrist)
Some watches promise to measure body fat percentage, muscle mass, and body water via wrist-based bioimpedance. The accuracy of these measurements is extremely low. Full-body bioimpedance scales already have significant margins of error — measuring from the wrist adds even more imprecision. It’s entertainment, not health data.
”Readiness score”
This metric combines sleep, HRV, recent activity, and other data into a single number that supposedly tells you whether you’re “ready” to train or should rest. The problem: each manufacturer uses a different proprietary algorithm, there’s no scientific standardization, and there’s no robust clinical validation for these scores.
The trap: when data becomes anxiety
There’s a growing phenomenon that researchers call orthosomnia — anxiety caused by the obsessive pursuit of perfect sleep scores. People who slept well start sleeping worse because they’re anxious about what number the watch will show in the morning.
This pattern can extend to any metric: compulsively checking heart rate, getting anxious because stress is “high,” feeling guilty for not hitting the step goal on a rainy day.
When the number on your wrist becomes more important than how you actually feel, the wearable has stopped being a tool and become a source of the problem.
Signs that the technology is hurting more than helping:
- You check the watch before assessing how you feel
- A “bad” score ruins your mood even when you felt fine
- You feel guilty on rest days
- You can’t sleep because you’re thinking about your sleep score
- You avoid activities the watch can’t track
How to actually use wearables well (and not be used by them)
The technology on your wrist can be a powerful ally — if used with intention. Here are five principles for turning data into real health.
1. Pick two to three metrics that matter for YOUR goals
If you want to sleep better, focus on sleep duration and quality. If you’re training for a race, track RHR and training minutes. Don’t try to monitor everything at once.
2. Look at TRENDS (weeks), not daily numbers
One bad night of sleep isn’t a crisis. An entire week of bad sleep is a pattern. Wearables are most useful when you look at the 7-to-30-day average, not today’s number.
3. Use the data to inform decisions, not dictate them
“My HRV has been low for a week, I’ll reduce training intensity” is a good data-informed decision. “My HRV is low today, I’ll cancel my workout” is reactivity to a single data point.
4. If the watch increases your anxiety, take it off
Seriously. No health metric is worth more than your mental health. If the act of tracking is generating more stress than clarity, take a break. Try a week without the watch and see how you feel.
5. Share data with your healthcare professional
Long-term trends — especially for sleep, RHR, and HRV — can be valuable information in a medical appointment. Your healthcare professional can interpret the data in the full context of your health, something no algorithm does on its own.
What actually matters
The best use of a wearable isn’t collecting numbers — it’s gaining awareness of patterns you wouldn’t notice otherwise. Are you sleeping less than you think? Is your resting heart rate trending up? Are you moving less during your most stressful weeks?
These are questions that data can help answer. But the final answer always comes from you: what are you going to do with that information?
Because at the end of the day, the most expensive watch in the world doesn’t replace the decision to go for a walk, get to bed earlier, or seek help when something isn’t right. Data is the beginning. Action is what changes health.