HRV explained simply.

What HRV measures

The gaps between two heartbeats are never exactly the same; they vary by a few milliseconds. Heart rate variability, HRV for short, sums up this variation in a single number. Among other things, it is linked to the shift between tension and relaxation over the day.

Watches and rings usually measure HRV at rest: at night while you sleep or in quiet moments during the day. A single reading is a snapshot; HRV only becomes meaningful over time.

SDNN, RMSSD – and why values are not comparable

There are several ways to calculate it. Apple Watch stores HRV as SDNN, while many other watches, rings and apps use RMSSD. On top of that, timing matters: an overnight average is something different from a short reading in the afternoon.

That is why 45 milliseconds on two devices do not mean the same thing – and comparing two people says little. What makes sense is comparing yourself with yourself, using the same source, over several weeks.

What many people see over time

HRV varies considerably from day to day. Many people see lower values after short nights, intense training sessions, late meals, alcohol, travel or especially busy phases – and higher values on quiet days. How strong this is varies a lot from person to person.

A single low value therefore says little. Looking across weeks shows more: where is your usual range, and are there recurring patterns, for instance after certain days?

HRV in mypacing

In mypacing, HRV is one of three recovery signals in the daily picture, together with resting heart rate and sleep. It is compared with your usual range over the last four weeks and described – without thresholds, without warnings, without a score.

Under “Patterns”, the app shows recurring links in your entries, always with the number of days they are based on. Where the values come from is explained in the guides on Apple Health and Health Connect.

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