What is a good HRV on Apple Watch? What your SDNN number means

There is no single good number. Apple Watch reports HRV as SDNN, in milliseconds, and healthy adults spread widely: in a study of 1,906 healthy people, average 5-minute SDNN fell from about 49-50 ms at ages 25-34 to about 28-30 ms at 65-74, with standard deviations of 12-21 ms (Voss et al., PLOS ONE, 2015). So 38 or 41 ms is ordinary for many adults. Whoop, Oura and Garmin report RMSSD, a different formula, so their numbers don't compare. The useful question is whether tonight's reading is in line with your own 7-night average, measured the same way.

By escapethematrix.app · Updated October 22, 2026

"Is 38 ms a good HRV?" is one of the most common questions people type after opening the Health app. A single number tells you very little on its own. What it means depends on how Apple Watch measured it, which formula it used, your age, and above all what is normal for you. This guide explains what the number is, why it moves, and how to read it without worrying over one bad night.

escapethematrix.app publishes Body Vitals, RepSquad and Presence, three iPhone apps. Body Vitals is mentioned once, near the end, and disclosed there. Everything else on this page is cited to Apple's own documentation or peer-reviewed research.

What Apple Watch actually measures

Heart rate variability (HRV) is the variation in time between heartbeats. Apple's HealthKit documentation says Apple uses SDNN: the standard deviation of the intervals between normal heartbeats, in milliseconds, recorded automatically on Apple Watch (Apple Developer).

Apple's technical paper on heart rate sensing adds how and when (Apple, 2024):

On most watches you get a handful of readings a day, taken whenever you happened to be still, so the times differ from day to day. A reading on the sofa after lunch and one during sleep are not measuring the same state.

What changed with Series 12 and watchOS 27

Apple Watch Series 12 and Ultra 4 measure HRV as often as every five minutes, which Apple says is 24 times more frequent than before (Apple Newsroom, September 2026). They also split HRV into Recovery HRV (daily stress and recovery signals) and overall HRV (broader health, including cardiovascular health). Overnight Vitals include Recovery HRV compared with your personal baseline.

On those models, Recovery HRV appears in the Heart Rate app, and the Vitals app needs about 7 nights of sleep wear to set your typical range (Apple Support, 120142). iOS 27 also adds an RMSSD HealthKit type (Apple Developer). Apple does not say which formula Recovery HRV uses; most Health app history is still SDNN.

SDNN vs RMSSD: why your Whoop, Oura or Garmin number differs

Both are time-domain HRV measures in milliseconds, but they capture different things (Shaffer and Ginsberg, 2017):

• SDNN is the standard deviation of all normal beat-to-beat intervals in a recording. Both branches of the nervous system contribute to it.
• RMSSD is the root mean square of successive differences between neighbouring beats. It reflects vagal ("rest and digest") influence more than SDNN does.

Most other wearables report RMSSD at night: Whoop during slow-wave sleep (Bellenger et al., 2021), Oura from sleep samples (Cao et al., 2022; Oura Help), Garmin continuously while you sleep (Garmin). An Apple Watch SDNN of 40 ms and an Oura RMSSD of 40 ms are different measurements, often at different times. Shorter recordings also tend to give smaller time-domain values, and 24-hour, 5-minute and shorter norms are not interchangeable (Shaffer and Ginsberg). There is no reliable conversion between devices. If you switch wearables, start a new baseline.

Is my number good? Putting it in context

Population averages are context, not targets. Voss and colleagues studied 1,906 healthy adults with 5-minute resting ECGs (PLOS ONE, 2015). Average SDNN fell with age, and the spread around each average was wide. Here is what that means for numbers people often search (one standard deviation = average ± SD):

For every age group and both sexes, see the full HRV-by-age chart.

A broader review of 44 studies and 21,438 healthy adults (Nunan et al., 2010, tabulated in Shaffer and Ginsberg, 2017) found an average short-term SDNN of 50 ms (SD 16), with study averages from 32 to 93 ms. In the Voss study, SDNN and RMSSD did not differ significantly between women and men in any age-matched group; age mattered far more. Watch readings are not lab ECGs: treat the table as rough context. The rule that SDNN below 50 ms is "unhealthy" applies to 24-hour monitoring in cardiac patients, not to a wrist reading (Shaffer and Ginsberg).

Why HRV moves day to day

HRV is meant to move. Alcohol lowered an HRV-based recovery score in the first hours of sleep in a dose-dependent way in 4,098 employees (Pietilä et al., 2018). Illness, mental stress, hard training and short or late sleep all shift the number; a falling 7-day average tracked overreaching in elite triathletes (Plews et al., 2012). If nights have been short, see can you catch up on lost sleep.

What to do with it: a 7-day baseline

Athletes are monitored with multi-day averages rather than single readings (Plews et al., 2013). For Apple Watch: wear it to sleep, compare overnight with overnight, and use a 7-night average as your reference. One low night after a drink is normal; several nights well below your average, especially with a higher resting heart rate, is a signal to ease off. Sleep, regular activity and recovery days move HRV over time more than chasing a higher number.

If a sustained drop comes with chest pain, fainting, a racing or irregular heartbeat, or feeling unwell, see a doctor. Apple Watch HRV is not a diagnostic test.

How Body Vitals reads your HRV

Disclosure: we make Body Vitals. Body Vitals reads the SDNN your Apple Watch writes to Apple Health and compares last night with the average of your last 7 nights, on-device. HRV is the largest single input to its daily readiness score (30% by default), alongside sleep, resting heart rate, blood oxygen and training load. The basic readiness score and HRV widget are free. It does not diagnose anything. Download is free; some features need an in-app purchase (App Store).

Sources

1. Apple Developer Documentation. heartRateVariabilitySDNN and heartRateVariabilityRMSSD.
2. Apple. Using Apple Watch to measure heart rate, calorimetry, and activity. November 2024.
3. Shaffer F, Ginsberg JP. An overview of heart rate variability metrics and norms. Frontiers in Public Health. 2017;5:258. PubMed, full text.
4. Voss A, Schroeder R, Heitmann A, Peters A, Perz S. Short-term heart rate variability: influence of gender and age in healthy subjects. PLOS ONE. 2015;10(3):e0118308. DOI.
5. Pietilä J, Helander E, Korhonen I, et al. Acute effect of alcohol intake on cardiovascular autonomic regulation during the first hours of sleep. JMIR Mental Health. 2018;5(1):e23. PubMed.

This guide summarises published research and Apple's documentation. It is not medical advice. Apple Watch HRV is a wellness indicator, not a diagnostic test; if you have symptoms, a heart condition or a sustained unexplained change, talk to a doctor.

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