HRV: what heart rate variability tells you
HRV is the variation in time between heartbeats. RMSSD vs SDNN, why your own baseline beats any chart, and what pushes the number up or down.
What is HRV?
Heart rate variability (HRV) is the variation in time between consecutive heartbeats, measured in milliseconds. Even at a steady 60 beats per minute, beats are not exactly one second apart. Higher resting HRV generally reflects more parasympathetic, or rest-and-digest, activity. Values differ widely between people, so HRV is most useful compared with your own baseline, not anyone else’s.
Your heart doesn’t tick like a metronome. At 60 beats a minute, one gap might be 980 milliseconds and the next 1,030. That small wobble is heart rate variability. It comes mainly from the autonomic nervous system. The parasympathetic side, carried largely by the vagus nerve, slows the heart and adds variation. The sympathetic side, the fight-or-flight system, speeds it up and makes the rhythm more regular.
How it works
When you’re rested and calm, the parasympathetic side has more say, and HRV tends to be higher. Short sleep, alcohol, stress, illness or a very hard training day shift the balance, and HRV tends to drop. That’s why athletes and wearable makers use it as a rough read on recovery. On its own it isn’t a measure of fitness or health. It’s one signal that moves with many things.
Wearables estimate HRV with an optical sensor that reads blood flow at the wrist or finger. A chest strap reads the heart’s electrical signal and is usually more precise. Either way, accuracy is best when you’re still, which is why most devices measure during sleep or ask for a quiet reading.
RMSSD vs SDNN
| Metric | What it measures | Where you’ll see it |
|---|---|---|
| RMSSD | Root mean square of successive differences. Tracks beat-to-beat changes and mainly reflects parasympathetic activity. | Many rings, bands and sports watches, usually measured overnight. |
| SDNN | Standard deviation of all normal beat intervals in a window. Reflects overall variation from several sources. | Apple Health, from short readings the Apple Watch takes through the day and night. |
The two numbers aren’t interchangeable. The same person can show quite different values on different devices, and switching metric or device resets the comparison. Pick one source and stick with it. If you wear an Apple Watch, the HRV in Apple Health is SDNN.
Why your baseline matters more than the number
HRV varies a lot between healthy people. Age, genetics, fitness, sex and the device all play a part. Two healthy people of the same age can differ by a factor of two or more, and HRV tends to drift lower with age. Comparing your number with a friend’s, or with a chart online, tells you almost nothing. What matters is how today compares with your normal.
Day to day, HRV is noisy. A single low morning is common and usually means little. Coaches and researchers who use HRV tend to watch a rolling 7-day average against a longer baseline of several weeks. A sustained drop below your usual range is more informative than any one reading.
A worked example
Say your ring reports a nightly RMSSD that has averaged 55 ms over the past month, usually landing between 48 and 62 ms. After two glasses of wine on Friday, Saturday reads 41 ms. After a hard leg session on Tuesday, Wednesday reads 50 ms, inside your usual range. The Saturday dip has an obvious cause and is back to normal by Monday. If your 7-day average sat around 44 ms for two weeks with no clear reason, that would be worth a look: your sleep, your training load, your stress, and how you actually feel.
What moves HRV
- Sleep: Short or broken sleep tends to lower it the next morning.
- Alcohol: Even a couple of drinks often lowers overnight HRV noticeably.
- Illness: A cold or a fever often pulls it down while it lasts.
- Stress: Work pressure, travel and worry all show up.
- Training: A very hard session can lower HRV for a day or two. Over months, regular aerobic training tends to raise resting HRV.
- Timing: Readings depend on when and how they’re taken. Overnight or same-time morning readings are the most consistent.
Common mistakes
- Comparing with other people: Your own baseline is the only fair comparison.
- Reacting to one reading: Look at the trend over a week, not a single morning.
- Mixing devices: SDNN from one source and RMSSD from another aren’t the same number.
- Treating it as a medical test: It’s an estimate from a consumer sensor. If you have chest pain, fainting, shortness of breath or a racing or irregular heartbeat at rest, see a doctor.
How FORM uses it
FORM reads HRV from Apple Health or Health Connect, alongside resting heart rate and sleep from a watch or ring that syncs there. Its recovery score is measured against your own 14-day baseline, not against other people, and sorts the day into Low, Typical for you, Recovered or Primed. It needs a night tracked by a wearable. It’s a guide for planning training, not a medical reading.
FAQ
What is a good HRV?
There isn’t a single good number. HRV varies widely between healthy people by age, genetics and fitness, so the useful question is whether yours is in its usual range.
Why is my HRV lower than my friend’s?
HRV differs a lot between individuals and between devices. A lower number than someone else’s doesn’t mean you are less fit or less healthy.
Is HRV from Apple Watch accurate?
Apple Watch estimates HRV with an optical sensor and reports SDNN from short readings. It’s most useful for following your own trend, not as a precise measurement.
How can I raise my HRV?
Consistent sleep, less alcohol, managing stress and regular aerobic training tend to help over weeks and months. Day-to-day swings are mostly noise.