In one sentence
A heart-rate-variability figure for the whole day, built from the short readings your watch takes while you are awake, and reported by a different method depending on which health store it came through.
The catch, first
There are two common ways to turn a string of heartbeats into one HRV number, and they do not give the same answer. SDNN is the standard deviation of all the beat-to-beat intervals in the window and reflects every rhythm that moves the heart, slow and fast; RMSSD is the root mean square of the differences between neighboring beats and reflects the fast, breath-by-breath swing that the vagus nerve drives (Shaffer 2017). Apple Health reports SDNN; Google Health Connect reports RMSSD. The two are not interchangeable, and a daytime SDNN is not comparable with an overnight RMSSD from a ring even on the same person. That is why VT labels every HRV figure with its method and keeps the daily average apart from the overnight figure.
Why it matters
It is the one HRV figure available on a day you did not wear a sleep device, and a fall in it that agrees with a rise in daytime heart rate is a reading worth having. A watch takes HRV in brief samples a few times a day, at moments it chooses, so the daily average is a coarser instrument than the overnight series: it depends on what you were doing when the watch sampled.
What moves it
As for overnight HRV (alcohol, being unwell, hard training, stress and short sleep lower it; fitness and recovery raise it), plus the daytime noise: caffeine, posture, a meeting, a walk just before the sample.
How to measure it well
Wear the watch daily; do not chase the number. If you want an HRV series you can act on, the overnight one on a ring or band is the better instrument.
How Vitality Tracker uses it
A supporting reading in Symptom Radar (weight 0.4, lower is the direction it watches), compared with your own 30-day baseline. Your HRV comes from one source at a time; if you switch it, say from a watch (SDNN) to a ring (RMSSD), Symptom Radar does not yet start a new baseline by itself, so results are unsettled until the old readings roll out of the 30-day window. Capacity does not use it.
How far to trust the number
Trend-grade, and the least precise HRV figure VT holds: a few spot readings at unknown moments, by a method that differs by platform. Its direction over weeks is usable; a day is not.
Sources
- Shaffer F, Ginsberg JP. An overview of heart rate variability metrics and norms. Front Public Health 2017;5:258. doi:10.3389/fpubh.2017.00258
- Vitality Tracker, How it works: the HRV method on every reading.