Your HRV is 38. Your mate's is 90. Neither number means anything.
Apple Watch and most dedicated straps don't even measure the same thing. Here's why comparing HRV between people — or between devices — tells you precisely nothing, and what to compare instead.
Search "normal HRV by age" and you'll find charts. Tables of averages, banded by decade, presented as something you can measure yourself against.
They're close to useless, and acting on them is how people end up anxious about a number that was never about them.
The measurement problem nobody mentions
HRV is not one metric. It's a family of calculations performed on the same underlying data — the gaps between consecutive heartbeats — and different devices pick different members of that family.
| Metric | What it calculates | Who reports it |
|---|---|---|
| SDNN | Standard deviation of all intervals between beats | Apple Watch (HealthKit's native HRV type) |
| RMSSD | Root mean square of the difference between each beat and the next | Most dedicated straps and rings |
SDNN captures overall spread. RMSSD captures beat-to-beat change, which tracks parasympathetic activity more directly and is the one most fitness research uses.
An SDNN of 35 and an RMSSD of 35 describe different properties of the same heartbeat. There is no conversion factor between them. Comparing the raw numbers is like comparing miles per hour with minutes per mile — related, not the same.
Three more reasons your numbers won't match
Measurement window. Apple Watch takes spot readings opportunistically through the day. Straps and rings typically weight the overnight window, when you're still and there's far less movement noise. Same person, same night, two very different sampling strategies.
Sensor and filtering. Optical sensors at the wrist, at the finger, and electrical sensors at the chest have different error profiles. Each vendor applies its own artefact rejection.
Individual variation is enormous. Even with identical hardware and method, healthy adults span a very wide range. Genetics, age, aerobic fitness, breathing rate and posture all move it. A high absolute HRV is not a fitness trophy, and a low one is not a diagnosis.
So what is worth comparing?
Exactly one thing: today's reading against your own recent normal, measured the same way, by the same device, at the same point in your day.
That's it. That comparison carries real information. Every other comparison — to a chart, to a friend, to your old watch — carries none.
The practitioner convention is to work with a rolling seven-day average rather than raw daily values, because day-to-day noise is large. A deviation only becomes interesting when it's big relative to your normal variability — someone whose readings swing widely needs a bigger move before it means anything than someone whose readings are tight.
What a meaningful drop looks like
- One low morning after a hard session. Expected. That's the training working. It should be heading back toward baseline within 24–48 hours.
- Low and still falling across three or four days. Now it's worth acting on — this is accumulation, not a blip.
- Low with an obvious cause. Alcohol, a short night, illness, travel. The signal is real, the mystery isn't.
- A single dramatic outlier. Usually measurement error. Check the reading was taken properly before rearranging your week around it.
Switching devices restarts everything. New metric, new window, new sensor, new baseline. Your first fortnight on a new wearable is calibration. Don't compare it to your old numbers — they were a different measurement.
The practical version
Pick one device and wear it consistently overnight. Give it two weeks before reading anything into the numbers. Then watch the trend against your own baseline, not the value. And when it drops, ask whether it recovers within a couple of days — that single question separates normal training response from something worth changing your week for.
The number was never the point. The distance from your own normal is.
Veylan scores your HRV against you
Not a chart, not an age band. It compares each morning to your own log-transformed baseline — and won't do it at all until it has two weeks of your data. Then it rewrites today's session and tells you which signal drove the change.
Download on the App StoreVeylan is a training tool, not a medical device. Nothing here is medical advice. If a persistently abnormal reading coincides with genuinely feeling unwell, speak to a doctor rather than to an app.