Guide · HRV

Apple Watch HRV explained

Short answer

Apple Watch reports heart rate variability as SDNN, the standard deviation of the time between normal heartbeats, in milliseconds. Most recovery apps, Whoop and Oura use RMSSD instead, and SDNN changes with recording length, so an Apple Watch number can look low next to other devices or online charts without anything being wrong. Read it against your own baseline.

Last reviewed 12 October 20265 published sourcesNot medical advice

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What Apple Watch actually measures

Apple's HealthKit documentation is explicit: there are several ways to compute HRV, and HealthKit uses SDNN, the standard deviation of the intervals between normal heartbeats, typically in milliseconds. The samples are recorded automatically by Apple Watch (Apple HealthKit: heartRateVariabilitySDNN). You will find them in the Health app under Heart Rate Variability.

SDNN vs RMSSD

  • SDNN reflects overall variability from every source, including slower swings, so it grows as the recording gets longer.
  • RMSSD is the root mean square of successive differences between beats. It tracks fast, beat-to-beat changes and is the metric most recovery apps use.

Short-term (about 5 minutes), ultra-short-term (under 5 minutes) and 24-hour values are not interchangeable (Shaffer & Ginsberg, 2017). The same healthy population reads very differently depending on the window:

Same metric, different recording window (healthy adults, mean ± SD)
Measure and windowMean ± SD
SDNN, 24-hour recording141 ± 39 ms
SDNN, 5-minute recording50 ± 16 ms
RMSSD, 24-hour recording27 ± 12 ms
RMSSD, 5-minute recording42 ± 15 ms
24-hour values: Task Force, Circulation 1996. 5-minute values: normative figures compiled by Shaffer & Ginsberg 2017 from Nunan et al. 2010.

The 24-hour figures come from the 1996 Task Force standards (Task Force, 1996); the 5-minute figures are the short-term norms compiled by Shaffer and Ginsberg (Shaffer & Ginsberg, 2017) from Nunan and colleagues' review of 44 studies with 21,438 participants (Nunan et al., 2010).

Why your Apple Watch HRV can look low

  1. Different metric. An SDNN value is not comparable with an RMSSD value from Whoop, Oura or a chest-strap app.
  2. Different window. Many “good HRV” figures online come from 24-hour SDNN, which reads roughly three times higher than 5-minute SDNN in the same people.
  3. Age. Short-term SDNN averaged about 49–50 ms at ages 25–34 but 28–30 ms at 65–74 in one large study (Voss et al., 2015). A 60-year-old comparing themselves with a 25-year-old's number will always look low.
  4. Daily life. Sleep, alcohol, illness, hard training, stress, posture and breathing all move HRV from one reading to the next.

How to read your Apple Watch HRV

Compare your watch with itself. Look at weekly averages rather than single readings, notice when a run of low values lines up with poor sleep, illness or a hard training block, and treat a sustained, unexplained fall as the signal worth acting on. For age context, use the checker below or the full HRV by age and sex table.

Compare your HRV with people your age

Apple Watch shows HRV as SDNN in the Health app. Whoop and Oura report RMSSD-based values.

years
Sex
Metric
ms

Your SDNN

45 ms

Mean, women 35–44

45.4 ms

n = 259

Mean ± 1 SD

24.9–65.9 ms

Your SDNN of 45 ms is within one standard deviation of the mean for women aged 35–44 in Voss et al.'s 5-minute lying-down ECGs. Watch readings are taken automatically during your day and night, so they are not directly comparable — your own trend on the same device is the more useful signal.

Your HRV against your own baseline

Reps builds a rolling HRV baseline from your own Apple Watch history and shows where today sits relative to your normal, next to your daily readiness.

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Common questions

Does Apple Watch measure HRV with SDNN or RMSSD?

SDNN. Apple's HealthKit documentation says that while there are multiple ways of computing HRV, HealthKit uses SDNN: the standard deviation of the intervals between normal heartbeats, in milliseconds. Apple Watch records these samples automatically.

Why is my Apple Watch HRV lower than my Whoop or Oura HRV?

They are different measurements. Apple Watch reports SDNN, while Whoop and Oura report RMSSD-based HRV taken during sleep. The two metrics produce different numbers from the same heartbeats, and the recording window also changes the value, so compare each device only with itself.

What is a normal Apple Watch HRV?

There is no Apple-published normal range. For orientation, 5-minute resting ECG studies found average SDNN of about 49–50 ms at ages 25–34 and 28–30 ms at 65–74, with wide spreads. Watch samples are recorded differently, so your own trend is more meaningful than any table.

Sources

Figures on this page were checked against these publications on 12 October 2026.

  1. Apple. heartRateVariabilitySDNN. HealthKit developer documentation. developer.apple.com/documentation/healthkit/hkquantitytypeidentifier/heartratevariabilitysdnn
  2. Shaffer F, Ginsberg JP. An overview of heart rate variability metrics and norms. Frontiers in Public Health. 2017;5:258. doi.org/10.3389/fpubh.2017.00258
  3. Task Force of the European Society of Cardiology and the North American Society of Pacing and Electrophysiology. Heart rate variability: standards of measurement, physiological interpretation and clinical use. Circulation. 1996;93(5):1043–1065. www.ahajournals.org/doi/10.1161/01.CIR.93.5.1043
  4. Nunan D, Sandercock GR, Brodie DA. A quantitative systematic review of normal values for short-term heart rate variability in healthy adults. Pacing and Clinical Electrophysiology. 2010;33(11):1407–1417. doi.org/10.1111/j.1540-8159.2010.02841.x
  5. 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.org/10.1371/journal.pone.0118308

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