Apple Watch HRV vs Recovery HRV, and SDNN vs RMSSD

Short answer: Apple Watch measures heart rate variability (HRV) as SDNN, the standard deviation of the time between heartbeats over a short reading. Most research and many other wearables use rMSSD instead. Both are in milliseconds, but they are different measures, so compare your Apple Watch HRV only with your own past readings. Recovery HRV is a separate figure that Apple shows on Apple Watch Series 12 and Ultra 4.

What Apple Watch measures, and when

Apple Watch estimates the time between heartbeats with its optical sensor and stores HRV in Apple Health as heart rate variability SDNN, in milliseconds. Apple says HRV is sampled while you are still, so how many readings you get depends on how active you are. In practice you get a handful of readings a day, including some while you sleep, and opening a Breathe session in the Mindfulness app adds one.

You can see every reading on iPhone: Health › Browse › Heart › Heart Rate Variability › Show All Data.

SDNN vs rMSSD in plain words

	SDNN	rMSSD
What it is	How spread out the beat-to-beat intervals are over the whole reading	How much each interval differs from the one before it
Reflects	Overall variability over the reading	Mostly fast, beat-to-beat changes linked to rest and digestion
Depends on reading length	Strongly: longer readings give higher SDNN	Less so
Used by	Apple Watch	Most HRV training research, many other wearables and apps

Neither is “better” for everyday use. The practical point is that they are not interchangeable: a 45 ms SDNN on Apple Watch and a 45 ms rMSSD from a chest strap or ring are not the same thing. Charts of “normal HRV by age” almost always mix measures, devices and recording lengths, which is why a good HRV on Apple Watch is best defined by your own history.

Why most research uses morning rMSSD, and why SDNN trends still help

HRV-guided training studies (Plews et al. 2013, Vesterinen et al. 2016) measured rMSSD under fixed conditions, usually right after waking, so each day was comparable. Apple Watch readings are taken whenever you happen to be still, which adds noise.

You can recover most of that consistency yourself:

• Use readings taken while asleep. Conditions are similar every night.
• Average over 7 nights. One night moves a lot; a week is steadier.
• Compare with your own range. Your 60-night normal range tells you what is usual for you on this device.

Under those rules, changes in Apple Watch SDNN are useful for spotting a lower-than-usual week. They are not a medical measurement, and they are not a direct stand-in for the rMSSD values in research papers.

Recovery HRV vs HRV

On Apple Watch Series 12 and Apple Watch Ultra 4, Apple shows a figure called Recovery HRV alongside overall HRV. You can see it in the Heart Rate app by turning the Digital Crown, and on these models HRV also appears among your overnight metrics in the Vitals app. Apple's support pages describe Recovery HRV as a measure of the variation in time between heartbeats; they don't say exactly how it is calculated or how it differs from the regular HRV samples.

Two practical consequences:

• If you have a Series 12 or Ultra 4, compare Recovery HRV with past Recovery HRV and HRV with past HRV. Don't mix the two.
• If you have an older watch, you don't get Recovery HRV, but the regular HRV readings taken during sleep give you a comparable nightly signal.

How Competo uses Apple Watch HRV

Competo Readiness uses only the HRV readings taken during sleep, works on the log scale, builds your normal range from the last 60 nights (mean ± 0.5 standard deviations) and judges the 7-day average against it. It adds resting heart rate and sleep duration and gives one of three bands: Primed, Steady or Take it easier. It works on any Apple Watch that records HRV while you sleep. To try the maths on your own numbers, use the HRV normal range calculator.

FAQ

Does Apple Watch use SDNN or rMSSD?

SDNN. Apple Health stores Apple Watch heart rate variability as “heart rate variability SDNN”, in milliseconds.

Can I convert SDNN to rMSSD?

Not reliably for one person. They are related, but the relationship depends on the recording, so there is no formula that turns your Apple Watch SDNN into a chest-strap rMSSD.

What is Recovery HRV on Apple Watch?

A figure Apple shows on Apple Watch Series 12 and Ultra 4, in the Heart Rate app and among overnight vitals. Compare it only with your own past Recovery HRV.

How often does Apple Watch measure HRV?

Several times a day, whenever you are still, including while you sleep. A Breathe session in the Mindfulness app also records a reading. The number of readings varies with how active you are.

Which HRV reading should I track?

The readings taken during sleep, averaged over 7 nights. Daytime readings depend on what you were doing just before, which makes them much noisier.

Why is my Apple Watch HRV lower than my friend’s ring or strap?

Different devices often use different measures (SDNN vs rMSSD) and different measuring windows, and people differ widely anyway. Compare only with your own history on the same device.

Sources

• Apple Support. Monitor your heart rate with Apple Watch (HRV sampling, Recovery HRV).
• Apple Support. Track your overnight and daytime vitals with Apple Watch.
• Apple Developer. heartRateVariabilitySDNN.
• Task Force of the European Society of Cardiology and the North American Society of Pacing and Electrophysiology (1996). Heart rate variability: standards of measurement, physiological interpretation, and clinical use. Circulation, 93(5).
• Shaffer, F., & Ginsberg, J. P. (2017). An overview of heart rate variability metrics and norms. Frontiers in Public Health, 5.
• Plews, D. J., et al. (2013). Training adaptation and heart rate variability in elite endurance athletes. Sports Medicine, 43(9).
• Vesterinen, V., et al. (2016). Individual endurance training prescription with heart rate variability. Medicine & Science in Sports & Exercise, 48(7).

Related guides

• What is a good HRV on Apple Watch?
• Readiness on any Apple Watch
• HRV normal range calculator
• Activity rings explained

Not medical advice. Competo is independent and not affiliated with Apple.