RMSSD (HRV metric)

The number behind your HRV score

When your watch shows an HRV number, it's almost certainly showing you RMSSD, the Root Mean Square of Successive Differences. Don't let the name intimidate you. It simply measures how much the timing between your heartbeats varies from beat to beat.

Your heart doesn't tick like a metronome. At 60 bpm, the gaps between beats might be 980 ms, then 1 050 ms, then 1 020 ms. RMSSD captures that variability, and more variability is generally a good thing.

The arithmetic, in one paragraph

Take the interval between each pair of consecutive beats (the RR interval). Subtract each interval from the next one: 1 050 minus 980 is 70, 1 020 minus 1 050 is minus 30. Square those differences so the signs stop cancelling (4 900 and 900), average them (2 900), and take the square root: about 54 ms. That is RMSSD. Because it is built on differences between neighbouring beats, it responds to the fast, breath-to-breath modulation of heart rate, which is exactly the part the vagus nerve controls. Other HRV measures exist: SDNN looks at the spread of all intervals over a longer recording, and frequency measures such as HF power isolate the breathing band, but for short recordings RMSSD is the workhorse and the one wearables report.

Why RMSSD, specifically?

• It primarily reflects your parasympathetic (rest-and-recover) nervous system, so it's a direct window into recovery
• It's less affected by breathing patterns than other HRV metrics (Shaffer 2017)
• It can be reliably measured from short recordings, 5 minutes or even overnight averages, and a single minute gives a usable estimate
• It's the standard used by Polar, Garmin, Oura, Withings and most research

The most accurate at-home RMSSD comes from a chest strap like the Polar H10, which records every beat-to-beat interval directly rather than estimating from an optical wrist sensor. Optical sensors do well at night, when the wrist is still, and poorly during movement; that is why every serious wearable computes its HRV from sleep.

What's a "good" number?

This is deeply personal. A young, fit person might have RMSSD of 60 to 100 ms or more. A 55-year-old with a desk job might see 15 to 30 ms. Both can be perfectly healthy. Across the population, RMSSD falls with age, roughly halving between 25 and 65, and the pooled reference range for healthy adults runs from about 19 to 75 ms around a mean near 42 (Nunan 2010), a spread so wide that a fit 60-year-old routinely out-scores a sedentary 30-year-old. What matters most is your personal trend; a sustained decline from your baseline is far more meaningful than any single reading.

Why the number jumps around

RMSSD is noisy by nature. Night-to-night swings of 10 to 20 % are normal, and the biggest drivers are alcohol (down 20 to 30 % the next night), a hard session (down for a night), a short night, illness (down for days), and simply breathing pattern and sleep position. Two consequences: judge a 7-day average against your 30 or 60-day baseline rather than one night against another, and do not compare your number with anyone else's, or with your own from a different device. Garmin reports a 7-day rolling value, Oura and Polar a per-night one, and each brand's algorithm cleans the signal differently, so the same person can read 45 on one wrist and 60 on the other.

In MyBodyAI

RMSSD is the primary input for the Nervous System, Recovery, Immunity and Body Alert indices. The absolute level is scored on a fixed curve centred near 40 ms, close to the pooled healthy-adult mean, and then shift the score by where your latest night sits against your own recent baseline, so your scores reflect how you are doing, not how you compare to a 25-year-old triathlete. Published references report pooled values without age strata, so there is no table to place a night in, and the app does not pretend there is. For a quick read on your own number, try the HRV Decoder.