Why did my HRV change when I switched devices?
Because each device measures a different stretch of the night, in a different way. Your heart didn’t change overnight. A new device starts a new history, so give it two to three weeks before you compare anything.
People switch from a watch to a ring, or one band to another, and wake up to a very different number. It looks like news. Switching bathroom scales works the same way. A two-pound gap between the old scale and the new one isn’t weight you gained.
What each device actually reports
| Device | Measurement | Which part of the night |
|---|---|---|
| Apple Watch | SDNN | Samples through the day and night |
| Oura | RMSSD | Five-minute samples, averaged over the whole night |
| WHOOP | RMSSD | An average weighted toward your last deep sleep |
| Garmin | RMSSD | Five-minute windows, averaged across sleep |
| Polar | RMSSD | The first 4 hours after you fall asleep |
Moving between Apple Watch and any of the others changes the measurement itself, because SDNN and RMSSD are different numbers. Between the RMSSD devices, the window still differs, and HRV changes with sleep stage. Two WHOOP scientists made the same point in 2025: a fair comparison needs the same measurement, the same window and the same way of averaging.
The sensors differ too. Against a chest strap, five wearables were off by 6 to 16 percent on average. Apple Watch Series 9 and Ultra 2 came in about 8 ms low. In athletes, an optical sensor gave 81 ms on average where an ECG gave 104.
What we’d do
- Never join the old history to the new one.
- If you can, wear both for two weeks. Check that they move in the same direction after the same nights.
- Build a new baseline on the new device over two to three weeks.
- Moving the same device to a new spot, wrist to upper arm, counts as a new history too.
Connect an Apple Watch, Oura, WHOOP or Garmin and every night’s HRV lands in Gyroscope on its own, charted against your own normal. Apple Watch’s SDNN and the RMSSD from Oura, WHOOP and Garmin are kept apart and never averaged. Track it.
5 sources
- Dial MB et al., Validation of nocturnal resting heart rate and heart rate variability in consumer wearables, Physiological Reports, 2025pubmed.ncbi.nlm.nih.gov
- Grosicki GJ, Presby DM, Accurate comparison of wearables requires contextual equivalence, Physiological Reports, 2025pubmed.ncbi.nlm.nih.gov
- Apple Developer Documentation, heartRateVariabilitySDNN, checked 2026-11-10developer.apple.com
- O'Grady B et al., The validity of Apple Watch Series 9 and Ultra 2 for serial measurements of heart rate variability and resting heart rate, Sensors, 2024pmc.ncbi.nlm.nih.gov
- Ben-David K et al., Pulse rate variability is not the same as heart rate variability, Sports Medicine Open, 2026pubmed.ncbi.nlm.nih.gov
