What is HRV? Heart rate variability explained
Short answer: Heart rate variability (HRV) is the small variation in time between one heartbeat and the next, measured in milliseconds. It reflects how your autonomic nervous system is balancing stress and recovery. Compared with your own baseline, higher HRV usually means you're well recovered, and a sharp drop often follows hard training, alcohol, poor sleep or illness.
The short version
Your heart doesn’t beat like a metronome. At 60 beats per minute, the gaps between beats might be 0.95 s, 1.05 s, 0.98 s and so on. HRV measures that variation.
The variation comes from your autonomic nervous system:
- The parasympathetic (“rest and digest”) branch slows the heart and adds variation, especially with each breath.
- The sympathetic (“fight or flight”) branch speeds it up and makes beats more regular.
When you’re rested, the parasympathetic side has more influence, so HRV is higher. Under stress, from training, illness, alcohol or a deadline, it falls.
How HRV is measured
| Metric | What it is | Used by |
|---|---|---|
| RMSSD | Root mean square of successive differences between beats. Tracks short-term, breath-to-breath variation. | Most research, WHOOP, Oura, Garmin |
| SDNN | Standard deviation of all beat intervals in a recording | Apple Health |
They aren’t interchangeable. See RMSSD vs SDNN.
Why your baseline matters more than the number
Healthy adults range from under 20 ms to well over 100 ms. Age, genetics, fitness and sex all play a part. That’s why recovery tools compare you with your own rolling average, not a chart. In elite endurance athletes, tracking morning HRV against a personal baseline has been used to time hard training and spot when someone isn’t coping (Plews et al., 2013).
How to use HRV day to day
- Measure consistently: overnight is best, because it’s the same conditions every day.
- Watch the trend, not one day: a 7-day average smooths out noise.
- Act on big, sustained drops: several days well below your normal means reduce training load and prioritise sleep.
- Learn your triggers: tag alcohol, late meals and stress, and see which ones move your HRV. See how to improve HRV.
HRV on Apple Watch
Apple Watch records HRV automatically, mostly overnight. You can see it in Health → Heart → Heart Rate Variability. See how to see HRV on Apple Watch. Pulse Coach uses it as 50% of a daily recovery score, measured against your 60-day baseline.
Questions people also ask
Is higher HRV always better?
Within your own range, higher usually means better recovery. But HRV differs hugely between people, so comparing your number with someone else's says little. A sudden unusual spike can also follow illness or extreme fatigue, so look at trends.
What lowers HRV?
Common causes are hard training without enough recovery, alcohol, short or poor sleep, late heavy meals, psychological stress, dehydration, heat, travel and getting sick.
Can HRV predict illness?
HRV often drops and resting heart rate rises around the start of an infection, but HRV is not a diagnostic test. If you feel unwell, talk to a doctor.
Sources
- Shaffer F, Ginsberg JP (2017). An overview of heart rate variability metrics and norms. Frontiers in Public Health.
- Task Force of the ESC and NASPE (1996). Heart rate variability — standards of measurement. Circulation.
- Plews DJ et al. (2013). Training adaptation and heart rate variability in elite endurance athletes. Sports Medicine.
Not medical advice. This article is general information about wellness metrics. It is not a diagnosis or a substitute for professional care. If a reading worries you, or you feel unwell, talk to a doctor.