Interpret heart rate variability readings.
Heart rate variability is reported as RMSSD in milliseconds, and raw values are almost uninterpretable in isolation: they fall with age, vary several-fold between healthy individuals, and shift with measurement posture and time of day. Two transformations make them usable. The log transform compresses the long right tail into a 0-100 score, which is why nearly every consumer platform reports lnRMSSD rather than raw milliseconds. The deviation from your own rolling average is the actionable figure — a drop beyond about 15% is a genuine signal, while day-to-day movement inside 5% is noise.
HRV score
Score = ln(RMSSD) x 20 - 20, clamped to 0-100
Baseline deviation
Deviation % = (today's RMSSD - 7-day average) / 7-day average x 100
These figures are estimates based on population-average formulas and are intended for training and recreational planning only. They are not medical advice. Consult a qualified health professional before starting or changing an exercise, recovery or cold-exposure programme, particularly if you have a cardiovascular condition, are pregnant, or take medication that affects heart rate or thermoregulation.
Because absolute RMSSD varies enormously between healthy people and falls with age. Comparing your number to another person's tells you nothing useful, which is why the deviation from your own baseline is the figure to act on.
On waking, before getting up, in the same position every time, and before caffeine. Posture alone can change RMSSD by tens of milliseconds, so an inconsistent protocol produces a baseline that cannot be interpreted.
Not always. Unusually high readings sometimes accompany overreaching, and parasympathetic overshoot after very heavy loads can look like excellent recovery. Read it alongside resting heart rate, sleep and how you actually feel.