The variation in time between successive heartbeats. One of the strongest wearable proxies for autonomic nervous system health, training readiness, and recovery status.
HRV measures fluctuation in the time intervals between consecutive heartbeats — specifically R-to-R intervals. A heart beating at 60 bpm is not beating with mechanical precision; the intervals vary. Greater variability generally indicates a more adaptive autonomic nervous system.
HRV reflects the balance between sympathetic (fight-or-flight) and parasympathetic (rest-and-digest) nervous system activity. High HRV correlates with strong parasympathetic tone — the body's ability to efficiently regulate stress, recovery, and adaptation.
The clinical gold standard is ECG. Consumer wearables use photoplethysmography (PPG) — optical sensors detecting blood volume changes through the skin. The most reported metric is RMSSD (root mean square of successive differences), which emphasizes parasympathetic activity. Apple Watch uses SDNN (standard deviation of all R-R intervals).
Based on Shaffer & Ginsberg (2017) and Altini & Kinnunen (2021). Values in milliseconds.
| Age | Males — Median (ms) | Males — Range | Females — Median (ms) | Females — Range |
|---|---|---|---|---|
| 18–25 | 55 | 35–85 | 60 | 38–90 |
| 26–35 | 48 | 28–75 | 52 | 30–80 |
| 36–45 | 40 | 22–65 | 43 | 24–68 |
| 46–55 | 33 | 18–55 | 36 | 20–58 |
| 56–65 | 28 | 15–46 | 30 | 16–48 |
| 66+ | 22 | 10–38 | 24 | 12–40 |
Sources: Shaffer F, Ginsberg JP. Front Public Health. 2017. Altini M, Kinnunen H. JMIR Form Res. 2021.