WHOOP shows HRV as 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 measured while you sleep, and the value leans towards the last stretch of deep (slow-wave) sleep rather than the whole night. In an independent 2025 study against a chest-strap 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 authors rated WHOOP 4.0’s agreement as moderate and acceptable (Dial and colleagues, 2025).
Documentation checked: 20 October 2026. WHOOP updates its software often, so details can change.
What number WHOOP shows
WHOOP shows heart rate variability in milliseconds with each Recovery. Its developer documentation names the field for RMSSD in milliseconds, next to 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 , blood oxygen and skin temperature (WHOOP developer documentation). RMSSD is a standard measure of how much the gap between one heartbeat and the next changes.
The band reads your pulse with light (an optical, or PPG PPG An optical method that shines light into the skin and detects the pulse of blood with each heartbeat. Most watches, rings and phone-camera apps use it to estimate HRV. More about PPG , sensor). WHOOP says the WHOOP 5.0 and WHOOP MG also use ECG technology for some sleep measurements, HRV among them (WHOOP Support, accuracy). WHOOP processes the raw data with its own filters, which it does not fully publish.
When WHOOP measures
WHOOP takes its HRV from sleep. As described in the Dial study, the value is a dynamic average during sleep, weighted towards the last stage of slow-wave (deep) sleep each night. An earlier study of WHOOP 2.0 also describes it measuring HRV during slow-wave sleep (Bellenger and colleagues, 2021). WHOOP does not publish the exact weights.
The window changes the number, because HRV changes through the night. In a sleep-lab study of 15 healthy young men, the median RMSSD was 67.3 ms in slow-wave sleep and 79.7 ms in REM sleep, and the authors described slow-wave sleep as a stable, repeatable state for measuring the heart (Herzig and colleagues, 2018).
How to read it
WHOOP calculates Recovery when you wake up, as a percentage from 0 to 100%. The measurements that go into it are resting heart rate and HRV, plus blood oxygen and skin temperature on WHOOP 4.0. WHOOP does not publish how it weighs them (WHOOP developer documentation).
- Look at the HRV, not only the percentage. A Recovery can fall because of resting heart rate or temperature on a night when HRV rose.
- Use weekly averages. Single nights jump around. A 7-day average shows the trend.
- Compare with your own past values. A WHOOP value is an RMSSD from deep sleep, so it is not directly comparable with a 5-minute reading taken while awake.
Recovery scores explained sets Recovery next to the other makers’ scores, and which recovery score should I trust answers the question in its title.
How WHOOP compares with other devices
The Dial study followed 13 healthy adults for 536 nights at home, with a Polar H10 chest-strap ECG as the reference. For overnight RMSSD, WHOOP 4.0 had a concordance (CCC, where 1 means perfect agreement) of 0.94 and an average error (MAPE) of 8.17%. Oura Gen 4 scored 0.99 and 5.96%. For resting heart rate, WHOOP’s concordance was 0.91, with an average error of 3.00%.
The researchers could not copy WHOOP’s deep-sleep weighting, so they compared WHOOP’s value with an all-night ECG average instead. Even so, the authors rated WHOOP 4.0’s agreement as moderate and acceptable.
In the earlier WHOOP 2.0 study, agreement with the ECG 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 how strongly bad beats were filtered out. The bias was smallest, 1.66%, with a 200 ms filter and with WHOOP’s own filter (Bellenger and colleagues, 2021), which shows how much processing choices can move the number.
Because WHOOP and Oura average different parts of the night, wearing both gives two numbers for the same night. See why Oura and WHOOP show different HRV, and the HRV by device guide for the other brands.
What to watch out for
- A night can go unscored. WHOOP’s developer documentation says a Recovery can be marked unscorable, usually because there was not enough data for the night.
- Do not mix it with other devices. If you also wear a ring, compare the direction of change, not the values.
- Keep the band in the same place. If you move it from the wrist to another spot, treat it as a new series.
- Expect a step after a new band or an update. See can a software update change my HRV, and give a new band a few weeks before you compare it with the old one.
- Wrist readings during exercise are a different matter. Optical sensors lose beats when the wrist moves. See is wrist HRV as accurate as a chest strap.