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.
Five interpreter tools
Where do you stand?
Age44
SexMale
VO2 Max38 ml/kg/min
Square in the median for men his age.
See where your number falls
All five: VO2 max by age · HRV by age · Resting heart rate · Sleep duration · Recovery score
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
In ordinary walking, your step count holds up well in the research. Your calorie number and the 220-minus-age formula hold up far less 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 breath sensor. Each estimates breathing from its optical pulse signal, and how well that works 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 reads skin temperature, not core body temperature, and most report it as a change from your own baseline. That change turns out to be informative.
Explore Temperature
Start reading
Latest articles
Heart Rate Variability
What's the Difference Between HRV and Resting Heart Rate?
Resting heart rate counts beats per minute; HRV measures how much the gaps between beats vary. Why they usually move in opposite directions, which one wearables measure more accurately, and what large cohorts found when both were tracked.
Read the article
Sleep
What's the Difference Between Core Sleep and Deep Sleep?
Core is Apple Watch's name for stages N1 and N2, what other trackers call light sleep; Deep is stage N3. Why a night that is mostly Core is normal, and why the watch's most common error is scoring deep sleep as Core.
Read the article
Activity
Do Workplace Wellness Programs and Wearables Actually Work?
Randomized trials in nearly 5,000 and 32,974 US employees found no effect on medical spending, health or absenteeism. What wearable trials, burnout studies and vendor claims add.
Read the article
Blood Pressure
Blood Pressure by Age: What's Normal for Men and Women?
Normal blood pressure is under 120/80 at every adult age, but average readings rise with age. A chart of US measured averages by decade for men and women, how common high blood pressure is at each age, and the US, European and Canadian cutoffs.
Read the article
Resting Heart Rate
Is a Resting Heart Rate of 45 Too Low?
A resting heart rate of 45 is lower than about 95% of adults. The cardiology guideline that flags rates under 50 says the number alone is not a diagnosis. What changes that.
Read the article
VO2 Max
What Is a Good VO2 Max for a 70 Year Old?
In your 70s the median VO2 max is 24.4 ml/kg/min for men and 18.3 for women. In 1,565 Norwegians aged 70 to 77, readings below 26.5 for men and 22.2 for women went with higher mortality.
Read the article
Continuous Glucose Monitoring
How Accurate Is a CGM in People Without Diabetes?
A crossover trial found CGM read about 0.9 mmol/L above capillary blood and overstated time above range fourfold. Other studies put the gap much smaller.
Read the article
Recovery
Why a Low Readiness Score Isn't a Reliable Signal to Rest, According to the Research It's Modeled On
A systematic review found the heart rate changes readiness scores rely on can mean overreaching or genuine fitness gains, and resting HRV barely moved.
Read the article
Wearable ECG
What an Irregular Heart Rhythm Notification Actually Confirms, and What It Doesn't Yet
In the watch's own confirmatory research, only 34 percent of people notified of an irregular pulse were later found to have AFib on a medical-grade ECG patch.
Read the article
Blood Oxygen (SpO2)
A Blood Oxygen Reading of 90 While Sleeping: The Number and the Sensor's Own Margin of Error
A watch reading of 90 is an estimate. In pooled Apple Watch studies, single readings fell within about 4 points either side of the reference.
Read the article
Respiratory Rate
Why Camera-Based Breathing Rate Accuracy Is Still Unsettled
A review found camera-based breathing-rate work still rests on small public datasets and inconsistent camera setups. The accuracy question is not answered yet.
Read the article
Stress
Does Biofeedback Work for Chronic Pain?
A multicenter trial of 1,015 treatment participants and 950 placebo controls found electrodermal biofeedback reduced self-reported chronic pain (p<0.05).
Read the article
Temperature
Can Your Wearable's Temperature Sensor Actually Catch Ovulation or Illness Before You Feel It?
Wrist temperature picks up a real ovulation signal and, separately, a real infection signal. Those are two different bodies of evidence, not one claim.
Read the article
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 the evidence behind it on a fixed five-level scale, from Established to Unsupported. 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 Research Desk
Every claim we make about heart rate variability, rated against the studies behind it.
Updated
Open the file
RHR Research Desk
What the research does and doesn't support about resting heart rate as a health signal.
Updated
Open the file
Sleep Research Desk
The sourcing behind our sleep claims, including where the evidence is still thin.
Updated
Open the file
VO2 Max Research Desk
How consumer wearables' VO2 max estimates hold up against the reference research.
Updated
Open the file
ECG Research Desk
What the AFib-detection accuracy studies actually show, rated claim by claim.
Updated
Open the file
CGM Research Desk
The sourcing behind our CGM claims, including what's still thin for non-diabetic use.
Updated
Open the file
Recovery Research Desk
What the research says about composite recovery and readiness scores, rated claim by claim.
Updated
Open the file
Activity Research Desk
How accurate step counts, calorie estimates, and exercise heart rate really are, rated against the studies behind them.
Updated
Open the file
SpO2 Research Desk
How accurate wearable blood oxygen readings really are, including skin-pigmentation bias and overnight sleep-apnea screening, rated against the studies behind them.
Updated
Open the file
Respiratory Rate Research Desk
How wearables estimate breathing rate from heart data alone, and why accuracy differs by brand, rated against the studies behind it.
Updated
Open the file
Stress Research Desk
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
Open the file
Temperature Research Desk
What a wearable temperature deviation can and cannot tell you about illness onset and cycle timing, rated against the studies behind it.
Updated
Open the file
Blood Pressure Research Desk
How watch and ring blood pressure readings hold up against a cuff, by day, at night and after calibration, rated against the studies behind them.
Updated
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.
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.