Heart rate variability (HRV)

Heart rate variability is the variation in time between heartbeats. Google Health reports a nightly average HRV for the main sleep, in milliseconds, computed as RMSSD (the root mean square of successive differences between beats). Pulse reads that value through the Google Health API; it does not compute HRV from raw beats.

Recovery

55% of the score, the largest single input.

Health Monitor

Checked against your normal range: your baseline ± 2 of your usual night-to-night swings.

Illness signal

A drop against your 30 nights before counts towards the combined illness pattern.

Behaviour insights

Shown as the change in standard deviations from your baseline after a logged behaviour.

There is no universal good HRV. It varies a lot between people with age, fitness and genetics, so Pulse never grades your HRV against other people. It compares tonight with your own baseline, a running average of recent nights that clips extreme values. Above your baseline is usually a good sign; a run of nights below it is worth noticing.

A night that Fitbit could not stage has no HRV, so Recovery gets no score rather than a guess. HRV moves with alcohol, illness, late meals, heat and hard training, so one low night says little on its own; trends say more. A wrist sensor's HRV is an estimate and is not comparable with a chest strap's morning reading.

1. Google Health API reference: data points and field definitions
2. Plews DJ, et al. Training adaptation and heart rate variability in elite endurance athletes. Sports Med 2013;43:773-81
3. Buchheit M. Monitoring training status with HR measures: do all roads lead to Rome? Front Physiol 2014;5:73
4. Altini M, Plews D. What is behind changes in resting heart rate and heart rate variability? Sensors 2021;21(23):7932