Which health metrics actually matter for longevity? (My ranking, with the evidence)

If you only track a few things for a long life, track these: your cardio fitness (VO₂max), how much you move every day, your resting heart rate, and how regular your sleep is. Those four have the strongest evidence behind them and your watch can see all of them. HRV matters less for longevity than its reputation says. And some of the biggest ones, like smoking, blood pressure and strength, no watch measures at all.

That's the short version. Below is my ranking, the study behind each one, and how much I trust the watch to measure it.

Why this question is harder than it looks

Your watch gives you maybe 40 numbers. Every one of them comes with an article saying it predicts how long you live. That can't all be equally true.

Two things to keep in mind before the list. First, almost all of this research is observational. It shows that people with a higher VO₂max live longer, not that raising yours buys you a specific number of years. Second, a metric can be a great predictor and a useless thing to track, if your watch measures it badly or if you can't do anything about it. So for each one I look at three things: how strong the evidence is, how well the watch measures it, and whether you can change it.

My ranking

All five are associations from observational studies. Sources at the end of the page.

Here is why each one sits where it does.

1. Cardio fitness (VO₂max): the strongest one you can actually change

This is the one I would pick if I could only keep one. When Mandsager and colleagues followed 122,007 people who did a treadmill test at Cleveland Clinic (JAMA Network Open, 2018), the least fit quarter had five times the risk of dying during follow-up compared with the fittest 2%. And there was no ceiling. More fitness was better all the way up.

A bigger picture comes from a meta-analysis of 33 studies by Kodama and colleagues (JAMA, 2009): every 1 MET of extra fitness, which is about 3.5 ml/kg/min of VO₂max, went with about 13% lower risk of death from any cause. A few points on your watch is a real difference.

The catch is the measurement. Apple Watch estimates VO₂max from your heart rate during outdoor walks, runs and hikes. In a 2025 study (Lambe et al., PLOS ONE), it read on average 6 ml/kg/min lower than a lab test, with an average error of about 13%. So I would not trust the number to tell you where you stand. I would trust it to tell you which way you are going, as long as you keep doing the same kind of outdoor sessions.

And you can move it. More than anything else on this list. Some hard sessions a week, the kind where talking gets difficult, plus a lot of easy ones.

2. Daily steps: the one with the best measurement

Steps are boring, and I think that's exactly why they get underrated. In the biggest pooled analysis so far, Paluch and colleagues combined 15 studies with 47,471 adults (Lancet Public Health, 2022). Compared with the least active quarter, at a median of 3,553 steps a day:

• about 5,800 steps went with 40% lower risk of death,
• about 7,800 steps with 45% lower,
• about 10,900 steps with 53% lower.

The benefit levelled off around 6,000 to 8,000 steps for people over 60, and 8,000 to 10,000 for people under 60. So 10,000 is not a magic number. But going from 3,000 to 7,000 is one of the biggest things anyone can do.

The good part: your watch counts steps very well, and you can change the number this afternoon. No other metric has both.

3. Resting heart rate: the quiet one you should trust

Resting heart rate doesn't get much attention because it doesn't move much from day to day. That's actually its strength. A meta-analysis of 46 cohort studies (Zhang et al., CMAJ, 2016) found that every 10 extra beats a minute at rest went with 9% higher risk of death from any cause. People above 80 beats a minute had about 45% higher risk than the lowest group.

It is also the number your watch measures best. When O'Grady and colleagues compared Apple Watch with a chest strap (Sensors, 2024), resting heart rate was off by 0.08 beats a minute. That's nothing.

Two things I'd add. A low resting heart rate mostly comes from being fit, so it overlaps a lot with number 1. And a resting heart rate that is creeping up over weeks, without more training to explain it, is the one I would not ignore.

4. Sleep: regularity first, then duration

Most people track how long they sleep. The research says when you sleep might matter more. In the UK Biobank, Windred and colleagues measured sleep timing with wrist sensors in 60,977 people (Sleep, 2024). Compared with the least regular sleepers, the more regular groups had 20 to 48% lower risk of death, and regularity predicted it better than sleep duration did.

Duration still matters, just in a U shape. In a meta-analysis of 1.38 million people (Cappuccio et al., Sleep, 2010), short sleep went with 12% higher risk and long sleep with 30% higher. The long-sleep part is probably more a sign of being ill than a cause, which the authors say themselves.

What your watch is good at: when you fell asleep and when you woke up. What it is much worse at: sleep stages. So for longevity I would look at your bedtime and wake time across a month, not at how much deep sleep you got last night. I wrote more about why timing matters so much in why you are tired after 8 hours of sleep.

5. HRV: real, but not the longevity number people think

I know this will surprise some people, because HRV is the metric everyone talks about. The evidence is real. A meta-analysis by Hillebrand and colleagues (Europace, 2013) pooled eight studies with 21,988 people without heart disease, and low SDNN, which happens to be the same measure Apple Watch uses, went with about 40% higher risk of a first heart attack or stroke.

But those studies measured HRV with an ECG, in controlled conditions. Your watch takes short spot readings whenever you are still, and in the same O'Grady study it read about 8 ms lower than the chest strap. That's fine for seeing your own trend from one week to the next. It is not good enough to say anything about your lifespan.

So I use HRV for what it is good at: telling me how my body is handling this week. Not for how long I'll live. If you want to read it properly, I explained how in how to read your HRV on Apple Watch.

What your watch can't see (and that matters just as much)

Some of the biggest numbers for a long life never show up on your wrist.

• Smoking. Bigger than anything above. No sensor needed to tell you what to do with that one.
• Blood pressure. A cuff at home, a few times a year.
• Strength. In the PURE study of 139,691 people in 17 countries (Leong et al., Lancet, 2015), every 5 kg lower grip strength went with 16% higher risk of death, and it predicted death better than blood pressure did. Your watch counts a strength session as a workout. It has no idea how strong you are.
• Waist and weight. A tape measure still beats every wearable.

This is the part I think most longevity apps skip, because it doesn't fit on a dashboard. But if you smoke, no VO₂max will make up for it.

What I would stop watching

For longevity, not for daily recovery:

• Last night's deep sleep percentage. Watches are not good at stages, and one night says nothing about ten years.
• Calories burned. A rough estimate, and it mostly tells you how much you moved, which steps already say better.
• Today's HRV or readiness score. Great for deciding today's workout. Meaningless for your lifespan on its own.

The pattern is the same as everywhere else: one day is noise. For longevity, think in months.

If I had to pick three

Honestly, if someone asked me what to look at once a month, I'd say:

1. Is my VO₂max going up or down over the last three months?
2. What's my average daily steps this month, and is it above 7,000?
3. Am I going to bed and getting up at roughly the same time, weekends included?

Then check that resting heart rate is not creeping up. That's it. Everything else is detail.

Where Woah fits

This ranking is basically how I had to think when I built Woah's longevity side.

Woah's fitness age takes your sleep, training, resting heart rate, HRV, weight and habits, works out what they add up to in years, and moves your real age by that much. On purpose, it ignores everything you can't change, like family history, because a number you have no way of moving is pointless. The longevity estimate starts from national life expectancy for your age and sex and adjusts it by what Woah can see, with a list of the changes that would move it most, each priced in days.

Neither one is a blood test or a prediction. The app says "an estimate, not science" right on the screen, and I mean it. The point is that it moves when you change something, by an amount you can understand. All of it is computed on your phone and free. What you pay for is the written part: a weekly note on what to change, and the coach. The FAQ explains how the numbers are built.

Questions people ask

Petter Höglander builds Woah. Before that he spent years building sleep and fitness apps for some of the biggest apps in the App Store. He wrote the recovery, sleep and longevity models Woah runs on, and he checks his steps more often than his HRV.