Documentation checked: 21 October 2026. Apple, WHOOP and Oura change their apps, plans and models often.
How Apple Watch, WHOOP and Oura measure HRV
Apple Watch: SDNN in Health, plus Recovery HRV
Apple’s developer documentation says Health stores Apple Watch HRV as SDNN SDNN The standard deviation of all NN intervals in a recording, in milliseconds. It measures overall variability, and its value depends strongly on how long the recording lasts. More about SDNN , the standard deviation of the intervals between normal heartbeats, and that the system records these samples on the watch automatically (Apple Developer Documentation). Each sample is a separate short reading, and Apple describes no way to start one. In validation studies, researchers used the Breathe app, now part of Mindfulness, to get readings from the watch (Hernando and colleagues, 2018; O’Grady and colleagues, 2024).
Series 12 and Ultra 4 add Recovery HRV. Apple says the watch now measures HRV as often as every five minutes and that overnight vitals compare Recovery HRV with your personal 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 (Apple Newsroom). The watch samples Recovery HRV while you are still, so the number of readings depends on how active you are (Apple Support). Apple has not said which statistic Recovery HRV uses. HealthKit for iOS and watchOS 27 lists a new data type named for 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 , but its page has no description (Apple Developer Documentation). watchOS 27 runs on Series 9 or later, SE 3 and Ultra 2 or later; Apple lists Recovery HRV only for Series 12 and Ultra 4. Recovery HRV vs overall HRV on Apple Watch explains the two numbers.
WHOOP: RMSSD weighted to deep sleep
WHOOP shows HRV in milliseconds with each Recovery, and its developer documentation names the field for RMSSD (WHOOP developer documentation). The authors of a 2025 validation study describe the value as a sleep average weighted towards your last slow-wave (deep) sleep stage (Dial and colleagues, 2025). WHOOP does not publish the exact weights.
Oura: RMSSD averaged across the night
Oura measures HRV only while you sleep. It takes 5-minute samples through the night and shows their mean as your average HRV, next to the highest single sample (Oura Member Care). The Dial study authors report that Oura, like WHOOP, shows RMSSD.
Why Apple Watch HRV differs from WHOOP and Oura
SDNN and RMSSD measure different things
SDNN is the spread of all the gaps between heartbeats in a recording. RMSSD is the size of the step from one gap to the next (Task Force, 1996). A slow drift in heart rate, as you settle or change posture, raises SDNN and barely moves RMSSD; one extra beat does the opposite. SDNN also grows with recording length, and the Task Force advises against comparing SDNN values from recordings of different lengths.
SDNN and RMSSD on the same beats
Both by the 1996 Task Force definitions
Gap before each beat 750 to 1050 ms
- SDNN
- 76.7 ms
- RMSSD
- 45.9 ms
- SDNN ÷ RMSSD
- 1.67
- SD1
- 32.5 ms
Beat-to-beat swings from breathing: both measures see them.
Here SDNN is 76.7 ms and RMSSD is 45.9 ms, a ratio of 1.67. For one slow breath the ratio was 1.67, so this series sits close to it. The ratio changes with the pattern of the beats, which is why no formula converts one into the other. SD1, by contrast, is always RMSSD divided by the square root of two: 32.5 ms here.
The presets and the text box work when JavaScript is on.
A line chart of 12 gaps between heartbeats with a dashed line at their mean, and readouts for SDNN, RMSSD, their ratio and SD1. Preset buttons load a slow breath, a slow drift, steady beats and one extra beat; a text box lets the reader type their own gaps. For the slow breath, SDNN is 76.7 ms and RMSSD is 45.9 ms. For the slow drift SDNN stays similar while RMSSD falls to a fraction; for the extra beat RMSSD jumps far more than SDNN.
Try “Slow drift”: SDNN keeps most of the spread while RMSSD falls to a fraction. No formula turns an Apple Watch SDNN into a WHOOP or Oura RMSSD; can I convert SDNN to RMSSD shows why.
Each device reads a different part of the day or night
HRV changes with sleep stage: in a sleep-lab study of 15 healthy young men, median RMSSD was 67.3 ms in slow-wave sleep and 79.7 ms in REM sleep (Herzig and colleagues, 2018). WHOOP leans on the last deep-sleep stage and Oura averages the whole night, while Apple Watch takes separate short readings through the day and night. The model below draws one simulated night and shades the part each device reads.
Same night, different numbers
A teaching model, not a measurement
Sleep stages lights out to waking
HRV every 5 minutes (RMSSD) 20 to 60 ms
- Apple Watch
- 4 readings, 48.7 to 78.9 ms SDNN; on waking 78.9 ms
Separate short readings the watch takes on its own, stored as SDNN. The number shown is the reading on waking.
Same night, same heart: WHOOP 54.6 ms and Oura 47.5 ms, because each device averages different hours. Apple Watch's short readings run from 48.7 to 78.9 ms SDNN, a different statistic.
The buttons work when JavaScript is on. The window for each device is listed in the description.
Two charts over eight hours of simulated sleep. The top chart is a hypnogram stepping between awake, REM, light and deep sleep, with deep sleep mostly in the first half and REM mostly in the second. The bottom chart is a line of HRV values, one per five minutes, that rise gently through the night and dip during wakeful minutes. One button per device shades the part of the night that device averages. Apple Watch: 4 readings, 48.7 to 78.9 ms SDNN; on waking 78.9 ms. WHOOP: 54.6 ms RMSSD. Oura: 47.5 ms RMSSD. Same night, same heart: WHOOP 54.6 ms and Oura 47.5 ms, because each device averages different hours. Apple Watch's short readings run from 48.7 to 78.9 ms SDNN, a different statistic.
Why Apple Watch HRV is different from other devices sets the watch beside other rings and bands.
Which is most accurate for HRV?
For overnight HRV, WHOOP and Oura have been tested against 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 more often than Apple Watch. In the only study we found of all three, on older models in a sleep lab, WHOOP 3.0 agreed almost perfectly with the ECG; Apple Watch Series 6 and Oura Gen 2 agreed less closely and both read about 10 ms low on average.
The one study that tested all three
Miller and colleagues fitted 53 healthy young adults (average age 25) with six wearables for one night in a sleep laboratory and compared overnight RMSSD with the lab’s ECG (Miller and colleagues, 2022).
Overnight RMSSD against a sleep-lab ECG
Miller and colleagues, 2022
53 young adults, one night each; the study’s Polar and Somfit results and Garmin’s test while awake are not shown. WHOOP gave the researchers its beat data; the Apple Watch values came through a third-party app and the Oura values from its app.
Source: Miller and colleagues, Sensors, 2022. Every study we have checked, with its numbers, is in the device validation tracker.
A dot chart in two panels, one row per device from the overnight test by Miller and colleagues (2022) against the ECG of a sleep-laboratory recording. The first panel shows the intraclass correlation (ICC), where 1 is perfect agreement; the second shows the average difference from the ECG in ms, where 0 is no difference and below 0 means the device read low. WHOOP 3.0: ICC 0.99, average difference −4.5 ms. Apple Watch Series 6: ICC 0.67, average difference −9.6 ms. Oura Ring Gen 2: ICC 0.63, average difference −10.2 ms. 53 young adults, one night each; the study’s Polar and Somfit results and Garmin’s test while awake are not shown. WHOOP gave the researchers its beat data; the Apple Watch values came through a third-party app and the Oura values from its app.
The limits of agreement show how far one person’s value could stray. For 95% of people, Apple Watch’s value would fall within 55.2 ms either side of its average difference from the ECG, Oura’s within 77.2 ms and WHOOP’s within 7.6 ms.
The devices did not hand over their data the same way. WHOOP gave the researchers its beat-to-beat intervals for the whole night, and the researchers worked out HRV themselves. The Apple Watch values came through Sleep Watch, a third-party app, and Oura’s from its own app; for both, the study notes that the stretch of the night the app sampled was not specified. The authors state that their research group receives funding and equipment from WHOOP, and that WHOOP was not involved in the design, conduct or reporting of the study.
Studies that tested one device at a time
These studies tested one brand, or two, in different settings, so their numbers do not rank the three.
| Study | Device | Setting | Result |
|---|---|---|---|
| Dial and colleagues, 2025 | Oura Ring Gen 4 | 13 adults, 536 nights at home, against a Polar H10 chest strap | CCC 0.99, MAPE 5.96% |
| Dial and colleagues, 2025 | WHOOP 4.0 | Same study | CCC 0.94, MAPE 8.17% |
| O’Grady and colleagues, 2024 | Apple Watch Series 9 and Ultra 2 | 39 adults, 316 five-minute morning readings lying down, against a Polar H10 | SDNN 8.31 ms low on average, MAPE 28.88% |
| Hernando and colleagues, 2018 | Apple Watch (beat intervals) | 20 adults sitting at rest and during a mental task, against a Polar H7 | HRV measures close to the strap’s; about 10% of intervals missing |
CCC is concordance, where 1 is perfect agreement; MAPE is the average percentage error.
In the 2025 home study, which did not include Apple Watch, the difference in percentage error between the Oura rings and WHOOP was not statistically significant; WHOOP vs Oura covers it in full. The 2024 study tested the SDNN that Apple Health shows. The 2018 study tested the beat intervals themselves, sitting, on an older watchOS.
Limits of the evidence
Each study here was small and in healthy people. None tested Apple Watch Series 12, Recovery HRV, WHOOP 5.0 or MG, or Oura Ring 5. All three companies use private algorithms that can change with an update; see can a software update change my HRV. The device validation tracker lists every study with its figures.
Apple readiness vs WHOOP Recovery vs Oura Readiness
Apple readiness, on Series 12 and Ultra 4, gives a score from 0 to 10 with one of four suggestions, from Recover to Go For It. Apple says it draws on recent activity, training load, vitals and sleep score, and updates through the day (Apple Newsroom).
WHOOP Recovery is calculated when you wake, from 0 to 100%. WHOOP lists 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 and HRV among its inputs (WHOOP developer documentation).
Oura Readiness sums contributors that include resting heart rate, body temperature, sleep, recent activity and HRV Balance, which compares a two-week average of your nightly HRV with your three-month baseline (Oura, Readiness Score Readiness score A single number, often out of 100, that a device builds from your HRV, resting heart rate, sleep and other inputs to suggest how ready you are for the day. The weighting is not published. More about Readiness score ; Oura, HRV Balance).
What is in a readiness score?
From the makers' own documentation
- HRV : named by the maker Vitals, including Recovery HRV analysed against your personal baseline.
- Resting heart rate : not named Not named by the maker.
- Breathing rate : not named Not named by the maker.
- Sleep : named by the maker Sleep score.
- Temperature : not named Not named by the maker.
- Activity or load : named by the maker Recent activity and training load.
Also: Apple's Vitals guide lists heart rate, breathing rate, blood oxygen (not on some US models) and wrist temperature among the vitals; Apple does not say how each counts.
Apple's Readiness names 3 of the six inputs: HRV, sleep and activity or load. Apple names four inputs and publishes no formula. It has not said which statistic Recovery HRV uses.
- HRV : named by the maker Heart rate variability; the API field for it is named hrv_rmssd_milli.
- Resting heart rate : named by the maker Resting heart rate.
- Breathing rate : not named Not named by the maker.
- Sleep : not named Not named by the maker.
- Temperature : named by the maker Skin temperature, for WHOOP 4.0 and later.
- Activity or load : not named Not named by the maker.
Also: Blood oxygen (SpO2) on WHOOP 4.0 and later. Sleep and breathing rate are not named in the developer documentation.
WHOOP's Recovery names 3 of the six inputs: HRV, resting heart rate and temperature. WHOOP lists the measurements that "factored into the score" but not how they are combined.
- HRV : named by the maker HRV Balance: "How has my HRV changed over the past few weeks?" (a two-week average against three months).
- Resting heart rate : named by the maker Resting Heart Rate: "Was my RHR substantially higher or lower than usual?"
- Breathing rate : not named Not named by the maker.
- Sleep : named by the maker Sleep, Sleep Balance, Sleep Regularity and Recovery Index (hours of sleep after the heart rate reached its low).
- Temperature : named by the maker Body Temperature: "Is my body temperature higher or substantially lower than usual?"
- Activity or load : named by the maker Previous Day Activity and Activity Balance (recent load).
Oura's Readiness Score names 5 of the six inputs: HRV, resting heart rate, sleep, temperature and activity or load. "The sum of these daily Readiness Contributors make up your Readiness Score." No weights are published.
A list of scores, one per device, with six inputs for each: HRV, resting heart rate, breathing rate, sleep, temperature and activity or load. Apple's Readiness names 3 of the six inputs: HRV, sleep and activity or load. Apple names four inputs and publishes no formula. It has not said which statistic Recovery HRV uses. WHOOP's Recovery names 3 of the six inputs: HRV, resting heart rate and temperature. WHOOP lists the measurements that "factored into the score" but not how they are combined. Oura's Readiness Score names 5 of the six inputs: HRV, resting heart rate, sleep, temperature and activity or load. "The sum of these daily Readiness Contributors make up your Readiness Score." No weights are published.
None of the three publishes its weights. A readiness of 6, a Recovery of 60% and a Readiness of 60 are not the same thing, and any of them can drop on a night your HRV rose, so read the HRV behind the score. Recovery scores explained lists each maker’s inputs, and which recovery score should I trust compares the scores.
Practical differences for tracking HRV
Where to find the number
- Apple Watch: the HRV samples are with your other heart data in the Health app on iPhone: tap Browse, then Heart (Apple Watch User Guide, heart rate). On Series 12 and Ultra 4, Recovery HRV is in the watch’s Heart Rate app: turn the Digital Crown (Apple Support).
- WHOOP: in each Recovery, in milliseconds.
- Oura: average HRV under Readiness and in Trends in the Oura app (Oura Member Care).
Where do I find HRV on my device lists other brands.
Wearing it to sleep and charging
A night without the device leaves a gap: WHOOP and Oura take HRV only from sleep, and Apple Watch checks Recovery HRV against your baseline in overnight vitals. Apple quotes up to 24 hours of everyday battery life for Series 12, where 15 minutes of charging adds up to 12 hours, and up to 50 hours for Ultra 4 (Apple Newsroom). If the watch has less than 30 percent charge at bedtime, it prompts you to charge it (Apple Watch User Guide, sleep schedules).
WHOOP’s battery packs slide onto the band and charge it while you wear it; its wired Basic Charger is not made to be worn (WHOOP Support, charging). The Oura ring comes off your finger to charge, so charge it while you are awake.
Membership
In its Series 12 announcement, Apple names a subscription only for Apple Fitness+ and Apple Music, and none for HRV, vitals or readiness (Apple Newsroom). WHOOP sells its band only as part of a membership, and every plan includes Recovery (WHOOP, membership). Oura sells the ring separately, but without an active membership the app shows only the Readiness, Sleep and Activity scores, and the nightly HRV is not among them; Gen2 rings need no membership (Oura Member Care, membership). Prices change, so we do not list them.
Getting your data out
- Apple Watch: Health can export all your health and fitness data as XML, and apps you allow can read it (iPhone User Guide). The RR converter turns an Apple Health export of beat intervals into a plain RR file.
- WHOOP: an in-app export sends CSV files, including one with Recovery, resting heart rate and HRV for each day (WHOOP Support, export).
- Oura: you can export all your data as CSV from the Membership Hub, with or without a membership (Oura Member Care, membership).
Can you wear Apple Watch with WHOOP or Oura?
Yes. Both the 2022 and 2025 studies had people wear several devices at once. Expect different numbers:
- Pick one as your main trend. Do not average them: an average of an SDNN and an RMSSD is not a measure of anything.
- Compare direction. Check whether the trends rise and fall on roughly the same days. A steady offset between them is normal.
- Start a new baseline when you switch. Keep a new device’s numbers apart from the old trend, and do the same when you move from an older Apple Watch to Recovery HRV. How long until a baseline is reliable lists the waiting times.
Which is best for tracking HRV?
For HRV tracking alone, the trade-offs are:
- Evidence. WHOOP and Oura have more overnight ECG studies behind them; no study we found has tested Recovery HRV.
- What the number is. WHOOP and Oura give one RMSSD per night. Apple Health’s SDNN comes from separate short readings of a length Apple does not document, which makes it harder to compare with published norms, and Apple has not described how Recovery HRV is calculated.
- Daily life. Apple names no subscription for HRV; Series 12 needs charging every day, and Ultra 4 lasts up to 50 hours. WHOOP charges on the wrist and comes with a membership. Oura is a ring, needs a membership for nightly HRV and comes off to charge.
You do not need any of them for HRV. A chest strap, the reference in most of these studies, reads the heart’s electrical signal; the chest-strap guide explains how.
For each device on its own, see Apple Watch HRV, WHOOP HRV and Oura HRV; HRV by device covers the other brands, and normal HRV explains what published values can and cannot tell you.
When to talk to a doctor
None of these devices diagnoses anything from HRV, and no readiness or recovery score is a medical test. Apple says its Vitals app is for wellness, not medical use (Apple Newsroom). Apple Watch also has a separate irregular rhythm notification, which checks your heartbeat now and then for a rhythm suggestive of atrial fibrillation (AFib); Apple advises talking to your doctor if you get one and have not been diagnosed with AFib (Apple Support, heart health notifications). Talk to a doctor if low HRV or low scores come with feeling unwell, lasting fatigue, palpitations or dizziness, whatever the number. Apple Watch cannot detect heart attacks: for chest pain, pressure or tightness, Apple advises calling emergency services (Apple Support, heart health notifications).