We interpret · You decide

Your wearable gives you the number. We read the research on what it means.

What's normal for someone your age, what moves your numbers over time, and where the science is still unsettled.

Real interpreter tools

Where do you stand?

VO2 Max38 ml/kg/min

Square in the median for men his age.

See where your number falls

The metrics

The numbers your wearable reports, explained honestly

Clear, sourced, and written to be useful

Heart Rate Variability What it says about your recovery and nervous system, and why one bad night isn't the whole story. Explore HRV Resting Heart Rate What's normal by age, what moves it over time, and what a rising trend actually means. Explore RHR Sleep Why your sleep score can be low on a night that felt fine, and what's actually being measured. Explore Sleep VO2 Max What counts as good for your age, and how much to trust your watch's estimate. Explore VO2 Max Wearable ECG What AFib detection actually catches, what it misses, and why an inconclusive reading isn't nothing. Explore Wearable ECG Continuous Glucose Monitoring What a glucose spike means if you don't have diabetes, and what the data can't tell you on its own. Explore CGM Recovery How composite recovery and readiness scores are built from HRV, RHR, and sleep, and how validated the score itself really is. Explore Recovery Activity Your step count holds up well in the research. Your calorie number and 220-minus-age formula don't hold up nearly as well. Explore Activity Blood Oxygen (SpO2) Overnight, your watch's blood oxygen reading holds up reasonably well. During the day, or if you have darker skin, the accuracy story gets a lot more complicated. Explore Blood Oxygen Respiratory Rate No wearable has a real breath sensor. Every one infers it from your heart rate, and how well that works genuinely differs by brand. Explore Respiratory Rate Stress Most stress scores are HRV wearing a different name. Only a couple of brands actually measure your skin's electrical response. Explore Stress Temperature Your wearable never reports a real temperature, only a deviation from your own baseline, and that deviation turns out to be genuinely informative. Explore Temperature

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Latest articles

Your Watch's VO2 Max Is a Formula. Does Estimated Fitness Still Predict Lifespan?

VO2 max's longevity link comes from treadmill tests. Research on formula-based estimates, and a real check on Apple Watch's own number, complicate that.

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Blood Oxygen (SpO2)

Can You Trust Your Watch's Blood Oxygen Reading at Altitude?

Hikers check their watch's SpO2 exactly when it matters most. Controlled altitude testing found accuracy falls apart right when you'd need it.

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Is Your Deep-Sleep Number Connected to Your Blood Pressure?

Insomniacs staring at a low deep-sleep score often wonder if it's hurting their heart. A 1,607-person study tested that exact link.

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Resting Heart Rate

Can Your Smartwatch See High Blood Pressure Coming?

Watches can't measure blood pressure directly, but a year-long study found their HRV and activity data predicted who developed hypertension.

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Heart Rate Variability

Does Cannabis Change Your Heart Rate and HRV More Than You'd Think?

Cannabis has a calming reputation, but a controlled THC study found heart rate climbed and HRV fell in a dose-dependent way, alongside more anxiety.

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Can You Screen Yourself for AFib by Feeling Your Own Pulse?

Cardiology guidelines endorse self-pulse checks for AFib. A study built to test that recommendation against real ECG readings ran in 1,010 patients.

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Continuous Glucose Monitoring

Does Your CGM Still Work Right at Altitude?

CGM sensors rely on oxygen-dependent chemistry, yet almost no one testing at altitude, on flights, or at elevation ever gets told that upfront.

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Why Exercise Training Doesn't Fix Everyone's Heart Rate Recovery

More training is supposed to mean faster heart rate recovery. In one clinical population, twelve weeks of aerobic training changed nothing at all.

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Do Fitness Trackers Accurately Count Steps in People With ALS?

A 2026 study tracked ALS patients for months with a Fitbit and a research-grade accelerometer side by side. Here's how closely they actually agreed.

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Respiratory Rate

Contactless Camera Respiratory Rate Accuracy: What the Research Actually Shows

A look at what systematic reviews say about camera-based respiratory rate tracking, where it holds up, and where the evidence base is still thin.

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Your Wearable Reads Electrodermal Activity for Stress. Can the Same Signal Be Used to Treat Pain?

A multicenter trial of ~2,000 people found electrodermal biofeedback significantly reduced perceived pain versus placebo, a different, active use of the same signal.

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Can Your Wearable's Temperature Sensor Actually Catch Ovulation or Illness Before You Feel It?

I looked at what wearable temperature sensors actually measure and what the research says about catching ovulation shifts or illness before symptoms hit.

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The Research Desk

Rated against the evidence, updated as it changes

Before we write that a number means something, we log the claim and rate it: established, uncertain, or contested. Each metric has its own file. These are our raw sourcing notes, not our articles. They're dense, built for checking our work. If you want the plain-English version, that's what the articles above are for.

HRV Evidence File Every claim we make about heart rate variability, rated against the studies behind it. Updated 7 days ago Open the file RHR Evidence File What the research does and doesn't support about resting heart rate as a health signal. Updated 7 days ago Open the file Sleep Evidence File The sourcing behind our sleep claims, including where the evidence is still thin. Updated 7 days ago Open the file VO2 Max Evidence File How consumer wearables' VO2 max estimates hold up against the reference research. Updated 7 days ago Open the file ECG Evidence File What the AFib-detection accuracy studies actually show, rated claim by claim. Updated 7 days ago Open the file CGM Evidence File The sourcing behind our CGM claims, including what's still thin for non-diabetic use. Updated 7 days ago Open the file Recovery Evidence File What the research says about composite recovery and readiness scores, rated claim by claim. Updated 7 days ago Open the file Activity Evidence File How accurate step counts, calorie estimates, and exercise heart rate really are, rated against the studies behind them. Updated 7 days ago Open the file SpO2 Evidence File How accurate wearable blood oxygen readings really are, including skin-pigmentation bias and overnight sleep-apnea screening, rated against the studies behind them. Updated 7 days ago Open the file Respiratory Rate Evidence File How wearables estimate breathing rate from heart data alone, and why accuracy genuinely differs by brand, rated against the studies behind it. Updated 7 days ago Open the file Stress Evidence File What separates a real electrodermal sensor from an HRV-proxy stress score, and how well either actually measures stress, rated against the studies behind them. Updated 7 days ago Open the file Temperature Evidence File What a wearable temperature deviation can and cannot tell you about illness onset and cycle timing, rated against the studies behind it. Updated 7 days ago Open the file

Most of what's written about these numbers is either vague or trying to sell you something. We read the actual studies and write down what they say, including the parts that aren't settled. You make up your own mind from there.

Kate Maren, Editor

Who we are

Three things we hold to

Independent. We don't sell a device, a supplement, or a program.
Evidence-led. Every claim traces back to a named study.
Interpretation only. We explain the research. We never tell you what to do.