Heart Rate Variability

Heart rate variability, usually shortened to HRV, is the variation in the time between one heartbeat and the next. A healthy heart does not tick like a metronome: the gaps lengthen and shorten constantly as your nervous system adjusts, and more variation generally means a body that is rested and responding well.

It is measured in milliseconds, and the number most devices report is RMSSD, the root mean square of the differences between successive beats. Typical readings run anywhere from the low twenties to well over a hundred, and the spread between two healthy people is far wider than the spread within one person over a month. That is why your own range is the only useful comparison.

What moves it is almost everything you would expect. Hard training, a late meal, alcohol, illness, poor sleep and stress all tend to push it down, and rest pushes it back up. A single low morning means very little; three or four in a row usually means something.

What it is not

It is not your heart rate, and the two often move in opposite directions: a night where your resting heart rate rose is usually a night where your HRV fell. It is also not a score, and a reading that is high for you may be unremarkable for someone else, which is why a figure without your own normal beside it is close to meaningless.

Where you see it in FitnessSyncer

Any service that measures HRV overnight sends it through to your Dashboard, where you can chart it over whatever window you like, and to your Stream. Your personal baselines compare each morning's reading against your own normal rather than against a population, and will tell you it is above, below or in line with that, which is a comparison rather than a verdict.