Apple 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, taken from short samples the watch records on its own rather than from one overnight average. That makes it a different number from the 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 most rings and bands report from sleep, so the two cannot be compared directly.
Documentation checked: 20 October 2026. Apple updates watchOS every year, so details can change.
Recovery HRV and overall HRV: see what each one is for.
What number Apple Watch shows
Apple’s developer documentation says HealthKit uses SDNN, the standard deviation of the beat-to-beat (RR) intervals between normal heartbeats, usually in milliseconds, and that the system records these samples automatically on Apple Watch (Apple Developer Documentation). You find them in the Health app under Heart, as Heart Rate Variability.
On the newest watches there is a second number. In its 9 September 2026 announcement of Apple Watch Series 12, Apple said the watch measures HRV up to 24 times more often and gives two versions: Recovery HRV, for daily signs of stress and recovery, and overall HRV, for broader health (Apple Newsroom). Apple’s support page says Recovery HRV is available on Apple Watch Series 12 and Apple Watch Ultra 4: open the Heart Rate app on the watch and turn the Digital Crown (Apple Support).
Apple has not said which statistic Recovery HRV uses. With iOS and watchOS 27, Apple’s HealthKit documentation also lists a new HRV data type named for RMSSD, but the page has no description and does not say whether it holds Recovery HRV (Apple Developer Documentation). Until Apple documents it, keep Recovery HRV in its own trend.
When Apple Watch measures
For Recovery HRV on Series 12 and Ultra 4, Apple says HRV is sampled while you are still, so the number of measurements you see varies with how active you are: you may get several readings on a quiet day and few on a busy one (Apple Support). For the SDNN in the Health app, Apple says only that the system records samples automatically (Apple Developer Documentation). Either way, they are separate readings, not one overnight average.
Overnight vitals on Series 12 now compare Recovery HRV with your own 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). Apple’s support page does not describe a way to start an HRV reading yourself. In a 2018 validation study, researchers used the Breathe app to get the beat intervals from the watch, the only way they found, and noted that the app stops when it detects repeated motion (Hernando and colleagues, 2018). That is why a Mindfulness (Breathe) session is the usual prompt for a reading.
How to read it
SDNN and RMSSD are different measures. SDNN includes slower swings in heart rate, and it depends heavily on how long the recording lasts, so SDNN values from recordings of different lengths should not be compared (Task Force, 1996). For your own data that means:
- Compare Apple with Apple. Look at your HRV trend over weeks in the Health app, not single readings.
- Expect swings within a day. Each sample is a short SDNN, and a slow drift in heart rate as you settle, a few deep breaths or a change of posture all raise it.
- Use a matching reference. If you look up norms, use SDNN values from a similar recording length, not RMSSD charts. Can I convert SDNN to RMSSD explains why there is no exchange rate.
- Keep Recovery HRV separate. Do not mix it into a trend of the older Health app values. Apple has published no ranges for Recovery HRV or overall HRV, so compare each with its own past values.
Apple’s new 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 runs from 0 to 10 and draws on recent activity, training load, vitals and sleep. It updates through the day, and Apple says the Vitals app is for wellness, not medical use (Apple Newsroom). Recovery scores explained sets it next to the other makers’ scores.
How Apple Watch compares with other devices
Apple Watch was not part of the 2025 Dial study, which tested Oura, WHOOP, Garmin and Polar devices 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 overnight. For the beat intervals, a 2018 study at rest was encouraging. In it, 20 healthy volunteers sat for five minutes watching a relaxing video and then did five minutes of a stressful attention test. The time-based HRV measures from the watch’s beat intervals did not differ significantly from those of a chest strap, and both showed RMSSD falling under stress. About 10% of intervals were missing from the watch recordings, in short gaps (Hernando and colleagues, 2018). The study was small, used an older version of watchOS, and tested people sitting still.
The number Apple Health shows did less well. In a 2024 study of Series 9 and Ultra 2, the watch’s own SDNN read on average 8.31 ms lower than a chest strap, with an average error of 28.88% (O’Grady and colleagues, 2024). In a 2022 sleep-lab study of 53 adults, overnight RMSSD from a Series 6, read through a third-party app, agreed only moderately with the lab ECG (ICC 0.67) and read on average 9.6 ms lower (Miller and colleagues, 2022). The device validation tracker lists both studies with their figures.
So when an Apple Watch and a ring disagree, the statistic and the timing often explain much of the gap, and the watch’s own error adds to it. Why Apple Watch HRV is different shows the difference between SDNN and RMSSD on the same beats.
What to watch out for
- Movement ends a reading. Optical sensors lose beats when the wrist moves, so a sample needs you to be still.
- A new watch or watchOS version can shift your values. See can a software update change my HRV.
- Do not mix brands in one trend. If you also wear a ring or band, keep its numbers separate.
- Your raw beats are available. The RR converter turns an Apple Health export of the beats into a plain RR file you can analyse yourself.