Yes. The HRV number in your app is the output of processing that the maker keeps private and revises, so an update can shift every night by a few milliseconds while your body stays the same. A step change on the date of the update, with your resting heart rate Resting heart rate How many times your heart beats per minute while you are fully at rest. For adults it is usually 60 to 100 beats per minute, and lower in well-trained athletes. More about Resting heart rate , sleep and how you feel unchanged, is the signature.
Why software can move the number
Between your pulse and the number on the screen sit several steps: detecting each beat from the optical signal, deciding which beats are real, filling or dropping the bad ones, picking which part of the night to use, and averaging. Each step has settings, and the settings are the maker’s to change.
A 2021 validation study of WHOOP shows how much one step matters. Bellenger and colleagues took the device’s beat intervals and cleaned them with WHOOP’s own filter and with several filter strengths in a research program, then compared the resulting RMSSD RMSSD Root mean square of successive differences: an HRV metric built from how much each heartbeat interval differs from the next, in milliseconds. It mainly reflects vagal activity. More about RMSSD with an ECG ECG A recording of the heart's electrical activity, taken with electrodes on the skin. ECG-based RR intervals are the reference standard for measuring HRV. More about ECG . Agreement for heart rate was good whatever the filter; agreement for log RMSSD lnRMSSD The natural logarithm of RMSSD, with no unit. Taking the log evens out the spread of RMSSD values, which makes day-to-day changes easier to compare and analyse. More about lnRMSSD depended on the filter, with the proprietary filter and the 200 ms setting doing best. Same beats, different cleaning, different HRV. An update that changes the cleaning changes your trend.
The 2025 overnight study of five wearables by Dial and colleagues makes the same point from the other side: the authors tested specific models and software, and call for continuous validation because the makers’ methods are private and move on. Garmin’s manual, for its part, does not say which formula it uses, so there is nothing to check an update against. HRV by device lists what each maker does publish.
Sometimes the definition changes, not just the number
In September 2026 Apple announced that Apple Watch now measures HRV up to 24 times more often and gives two kinds of HRV, a Recovery HRV for daily stress and recovery and an overall HRV for broader health, without saying which statistic Recovery HRV uses. A reader who sees a new, different HRV after that update is not looking at a shifted number but at a new one. Why Apple Watch HRV is different covers the two statistics.
How to tell a software shift from a real change
| Clue | Software | Your body |
|---|---|---|
| Shape of the change | A step on one date | A drift over days, or a dip that recovers |
| Other signals | Resting heart rate, breathing rate Breathing rate How many breaths you take a minute. It moves HRV on its own: slower, deeper breathing makes the beat-to-beat swing larger, and below about 9 breaths a minute it shifts from the HF band into LF. More about Breathing rate and sleep unchanged | They often move too |
| How you feel | The same | Tired, sore, unwell, or better rested |
| Other users | Report the same step after the same update | No pattern |
The simulator on how long a baseline takes has a “Firmware shift” button: every night reads about 6 ms lower from one date, the 7-night average falls out of the old band within a week, the status is flagged, and then the band slides down and the status settles. That is the whole episode, and nothing happened to the simulated body.
What to do after an update
- Write the update date next to your trend.
- Keep measuring the same way. Do not reset; a reset changes nothing except the wait.
- Compare new nights with new nights. After three weeks you have a new baseline Baseline Your own usual HRV level and range, built from weeks of readings taken the same way. Comparing a reading with your baseline says more than comparing it with other people. More about Baseline .
- If the status stays flagged beyond that and you feel fine, the shift was the software. If you also feel worse, read it as you would any low week: sleep, load, alcohol, illness.
When to talk to a doctor
A step in a device’s number is not a health event. Talk to a doctor if you have symptoms such as palpitations, fainting, chest pain or breathlessness, or if you feel unwell for weeks, whatever the app shows.