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What Is HRV (Heart Rate Variability)?
Heart rate variability (HRV) is the variation in time between one heartbeat and the next. Even at a steady 60 BPM, the gaps between beats are not all exactly one second. HRV is measured in milliseconds, mostly reflects the parasympathetic ('rest and digest') side of the nervous system, and is highly individual: it falls with age and differs a lot between healthy people.
Measure the time between pulses.
Measure from one pulse peak to the next, using the same reference point on each beat.
The standard definitions come from the 1996 Task Force of the European Society of Cardiology and the North American Society of Pacing and Electrophysiology. Time-domain measures work directly on the beat-to-beat intervals: SDNN is the standard deviation of all the intervals, RMSSD is the root mean square of the differences between successive intervals, and pNN50 is the share of successive intervals that differ by more than 50 ms. Frequency-domain measures split the variation into bands: very low frequency (up to 0.04 Hz), low frequency (0.04 to 0.15 Hz) and high frequency (0.15 to 0.4 Hz). The Task Force recommended 5-minute recordings for short-term analysis and 24-hour recordings for long-term measures, and warned that SDNN from recordings of different lengths should not be compared.
RMSSD is the number most wearables report, because it tracks fast, breathing-linked changes driven by the vagus nerve and works on short windows. Shaffer and Ginsberg's review notes that the LF/HF ratio, once read as a balance between the sympathetic and parasympathetic systems, is now contested, because low-frequency power has both sympathetic and parasympathetic inputs.
HRV falls with age. In a cross-sectional study of 8.2 million Fitbit users on one day in 2018, HRV measures decreased with age, and high-frequency power measured between 6 and 7 am fell by about 82% in men and 81% in women from age 20 to 60. Because HRV also varies widely between people of the same age, your own trend is more useful than a comparison with someone else.
Watches and rings estimate HRV from the pulse wave (photoplethysmography, PPG) rather than the heart's electrical signal, so strictly they measure pulse rate variability. Schäfer and Vagedes' review of these comparisons found agreement with ECG HRV good enough at rest, with short-term variability often slightly overestimated, but agreement worsened during physical activity and some mental stress. That is why most devices take HRV during sleep. In a 53-person sleep-lab study comparing six devices with ECG (Miller et al., 2022), overnight HRV agreement was excellent for WHOOP 3.0 (intraclass correlation 0.99) and good for Oura Gen 2 (0.63) and Apple Watch S6 (0.67), and each device sampled HRV over a different window.
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Frequently Asked Questions
What is a good HRV?
There is no single good number, because HRV depends heavily on age, the metric (RMSSD or SDNN), the recording window and the device. Compare yourself with your own baseline over weeks. For age-based reference ranges see our HRV by age chart and the good HRV score page.
Is a higher HRV better?
Across populations, higher resting HRV is generally associated with better health and fitness, and it falls with age. Day to day, a drop below your usual range can follow illness, alcohol, poor sleep or hard training, but a single reading on its own says little.
What is the difference between RMSSD and SDNN?
Both come from beat-to-beat intervals. RMSSD looks at differences between neighbouring beats and mainly reflects vagal (parasympathetic) activity; SDNN is the standard deviation of all intervals and reflects total variability. They are not interchangeable, and values depend on recording length.
How accurate is HRV from a watch or ring?
At rest and during sleep, PPG-based HRV agrees reasonably well with ECG; during movement it degrades. In a 2022 sleep-lab study, WHOOP 3.0 had excellent agreement with ECG for overnight HRV, and Oura Gen 2 and Apple Watch S6 had good agreement.