# RMSSD

RMSSD is the most common way to put heart rate variability into one number: the typical size of the change between one heartbeat interval and the next, in milliseconds.

It reflects the activity of the vagus nerve, which slows the heart at rest, and it works on short recordings of a few minutes. Wearables that report one HRV number usually report RMSSD or something close to it. The number differs between devices, so compare it only with your own readings on the same device.

Also called: Root mean square of successive differences.

Related: [HRV](https://revns.com/dictionary/hrv.md), [Resting heart rate](https://revns.com/dictionary/resting-heart-rate.md)

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Canonical: https://revns.com/dictionary/rmssd
Index of everything: https://revns.com/llms.txt
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