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.

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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

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Latest articles

Blood Oxygen (SpO2)

How Do Wearables Actually Measure Blood Oxygen?

A look at the light-based sensing behind wearable SpO2 readings, what the research says it measures well, and where it runs into physical limits.

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Does Wearing a Fitness Tracker Actually Make You More Active?

I looked at what the research actually says about whether strapping on a fitness tracker changes how much people move, not just how it measures them.

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Heart Rate Variability

Does HRV Reflect Depression?

HRV tends to run lower in depression at a group level, but research doesn't support reading a personal HRV number as a depression signal. Here's what's measured.

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Resting Heart Rate

Do GLP-1 Drugs Change Your Resting Heart Rate?

Wearable users on GLP-1 medications often see resting heart rate climb. Here's what a 12-week tracking study actually found, and what it didn't test.

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Does VO2 Max Protect Against Cognitive Decline?

Research links higher cardiorespiratory fitness to brain aging outcomes. Here's what the evidence actually establishes about VO2 max and cognitive decline.

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Does a Cuffless Blood Pressure Wearable Actually Work?

I looked at what the research actually says about cuffless blood pressure wearables, and the answer depends heavily on which device and which sensor.

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What Improves Recovery Scores

Recovery scores lean heavily on heart rate variability and resting heart rate. Here's what the research says actually shifts those inputs, and what it doesn't.

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Continuous Glucose Monitoring

What Does a CGM Show If You Don't Have Diabetes?

A look at what continuous glucose monitors reveal in non-diabetic wearers, and what the research says those glucose spikes and dips actually mean.

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When Your Sleep Score Becomes the Stressor

Orthosomnia, anxious preoccupation with sleep tracker scores, has been formally measured in a general population sample. Here's what that research found.

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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 6 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 6 days ago Open the file Sleep Evidence File The sourcing behind our sleep claims, including where the evidence is still thin. Updated 6 days ago Open the file VO2 Max Evidence File How consumer wearables' VO2 max estimates hold up against the reference research. Updated 6 days ago Open the file ECG Evidence File What the AFib-detection accuracy studies actually show, rated claim by claim. Updated 6 days ago Open the file CGM Evidence File The sourcing behind our CGM claims, including what's still thin for non-diabetic use. Updated 6 days ago Open the file Recovery Evidence File What the research says about composite recovery and readiness scores, rated claim by claim. Updated 6 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 6 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 6 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.