Your watch says 38. Your friend’s says 72.
For a moment, breakfast becomes a fitness competition. Is their heart doing something yours cannot?
Both numbers are labelled “HRV”, but you may be comparing different calculations, recording windows and bodies. The higher number does not establish who is healthier.
Heart-rate variability (HRV) measures how much the time between consecutive normal heartbeats changes. It differs from heart rate, which counts beats per minute.
To understand heart-rate variability, start with something your pulse display hides: the space between beats.
The space between beats: a conceptual illustration of HRV, not an ECG recording or product test.
Sixty beats a minute, with room for variation
A heart rate of 60 beats per minute sounds like one beat every second. But that average can conceal small differences.
Imagine five consecutive gaps between six beats:
1,000 → 970 → 1,030 → 990 → 1,010 milliseconds
The gaps average one second, equivalent to 60 beats per minute over this short sequence. Yet the timing varies. Another sequence could have exactly the same average with much less variation.
Heart rate tells you the pace. HRV describes variation in the intervals.
These are invented numbers to illustrate the idea, not a diagnostic sample. The average gap is not the HRV value, and neither is simply subtracting the shortest gap from the longest. Different HRV calculations capture different features of the pattern, as the Society for Psychophysiological Research’s guidance explains.
Why a healthy heart need not tick like a clock
Your heart’s natural pacemaker responds to the autonomic nervous system and other physiological influences. Breathing helps shape the rhythm: the heart commonly speeds up during inhalation and slows during exhalation.
A metronome repeats. Your heart adjusts.
This also explains why changing your breathing can change the reading during a recording. A bigger number after a breathing exercise does not, by itself, demonstrate a lasting improvement in health. The 2024 measurement guidance describes these relationships and their limits.
HRV offers information about cardiac regulation. It cannot tell you everything about your nervous system or identify a disease from a watch screen.
Two numbers can share a unit and mean different things
Look beneath the “HRV” label and you may find one of these:
| Measure | What it describes |
|---|---|
| RMSSD | Differences between successive normal beat intervals, emphasising short-term variation. |
| SDNN | How spread out the normal beat intervals are across the recording; recording length matters greatly. |
Both are commonly reported in milliseconds. They are not interchangeable.
Duration matters too. In an original study of 3,387 adults, agreement between short segments and longer recordings depended on the statistic and segment length. RMSSD performed better for the shortest segments. These were controlled resting measurements with signal cleaning, not proof that every quick wrist reading matches a longer recording.
Before comparing numbers, find out what was calculated, over how long, and when.
Your watch has to find the beats first
An ECG records the heart’s electrical activity. Optical wrist sensors use light to detect changes in blood volume and derive pulse timing.
That distinction matters because HRV depends on individual intervals. Movement, missed detections and abnormal beats can alter the result; how the software handles them matters too. The measurement guidance addresses these problems.
A tidy graph can hide a complicated measurement process. Check the recording instructions before giving every change a biological explanation.
So what counts as a good HRV?
An age chart looks reassuringly simple. Your physiology is less obliging.
In an original Framingham Heart Study analysis, age and heart rate were major determinants of the HRV measures examined. Those results came from ambulatory ECG recordings, so their reference values cannot simply be transferred to every watch statistic.
A useful reference range needs a matching population, calculation and recording method. “Above 50 is good” is not a universal rule.
Nor does your friend’s 72 make your 38 a failure. First establish whether the numbers are comparable.
Give your number some history
For everyday tracking, a personal baseline means your usual range across repeated, comparable recordings. Think of it as learning your own pattern before deciding that something has changed.
- Keep the method consistent: the same device, app and HRV statistic.
- Match the setting: overnight summaries with overnight summaries; resting spot checks with similar resting checks.
- For spot checks, follow the instructions: keep time, posture and breathing approach consistent.
- Allow repeated observations: several weeks can help reveal your pattern. Follow the manufacturer’s learning period where specified; there is no universal duration.
A new device or calculation method starts a new comparison. A jump on the screen may reflect the measurement change rather than a change in your body.
Your baseline helps you notice differences. It does not explain their cause.
Read the pattern, then choose the question
The useful question is: “How does this compare with my own recent readings, measured the same way?”
If you want to know what to do after a low reading or poor recovery score, our low-HRV and sleep-score guide covers that next step.
Normal beat-to-beat variation is also different from symptomatic rhythm problems. Recurrent palpitations deserve assessment. Palpitations with chest pain, breathlessness or fainting need urgent medical help, whatever the watch says. See the NHS palpitations guidance.
Before your next comparison, find your app’s HRV statistic and recording window. Then give your own pattern time to emerge. Your friend’s number can wait.
