Oura and WHOOP both measure RMSSD RMSSD Root mean square of successive differences: an HRV metric built from how much each heartbeat interval differs from the next. It mainly reflects vagal activity. More about RMSSD from an optical sensor while you sleep, but they average different parts of the night. Oura takes the mean of five-minute samples across your whole sleep, while WHOOP gives more weight to deep (slow-wave) sleep, so the same night can produce two different numbers.
Same statistic, different parts of the night
Both devices report RMSSD, a measure of how much the gap between one heartbeat and the next changes. The difference is which heartbeats go into the average.
- Oura measures HRV only during sleep. It takes five-minute samples throughout the night and shows their average as your nightly HRV, along with the highest value it recorded.
- WHOOP describes a “dynamic average” during sleep that is weighted towards the last period of slow-wave sleep, as quoted in a 2025 validation study. An earlier study of WHOOP 2.0 describes it measuring HRV during slow-wave sleep. WHOOP does not publish the exact weights.
HRV changes through the night. In a sleep-lab study of 15 healthy young men, each recorded on three nights 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 , the median RMSSD was 67.3 ms in slow-wave sleep, 70.7 ms in light (stage 2) sleep and 79.7 ms in REM sleep. The authors described slow-wave sleep as a stable, repeatable state for measuring the heart. An average that leans on deep sleep and one that covers every stage are measuring slightly different things.
Finger, wrist and data cleaning
Oura reads your pulse at the finger and WHOOP at the wrist (or upper arm). Both use light to detect the pulse, so neither sees the heart’s electrical signal directly. Each company also runs its own filter to remove bad beats before it calculates HRV. The WHOOP validation study found that the strength of that filter changed how closely WHOOP matched the ECG, which shows how much processing choices can move the number.
Is one of them wrong?
A 2025 study suggests both come close to a chest strap. It followed 13 healthy adults (mean age 33) for 536 nights at home, with a chest strap as the reference. For overnight RMSSD, agreement with the chest strap (as a concordance score, where 1 is perfect, and the average error) was:
| Device | Agreement | Average error |
|---|---|---|
| Oura Gen 4 | 0.99 | 5.96% |
| Oura Gen 3 | 0.97 | 7.15% |
| WHOOP 4.0 | 0.94 | 8.17% |
Because WHOOP’s weighting is not public, the researchers compared it with the chest strap’s whole-night average. Both devices came close. The study was small and only included healthy adults, so it cannot rank devices on its own. The HRV by device guide covers other brands.
What to do with your own data
- Choose one main device. Your trend from one device is more useful than mixing numbers from two.
- Compare direction, not size. If you wear both, expect a steady offset between them. Check whether both move up or down on the same days.
- Use weekly averages. Single nights jump around. A 7-day average shows the real trend.
- Rebuild your 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 when you switch. Give a new device a few weeks before you compare it with your old numbers. The lesson on building your HRV baseline explains how.
- Do not mix in daytime readings. A spot reading taken while awake is a different measurement again. See overnight vs morning HRV for the trade-offs.
An Apple Watch number differs for another reason, because it uses a different statistic. See why Apple Watch HRV is different.