SDNN

SDNN measures how far, on average, each NN interval NN interval NN interval An RR interval between two normal heartbeats, after extra beats and measurement errors are removed. Most HRV metrics are calculated from NN intervals. More about NN interval sits from the mean interval. A recording where the gaps between beats wander widely gives a high SDNN; one where they barely change gives a low SDNN.

Unlike RMSSD RMSSD RMSSD Root mean square of successive differences: an HRV metric built from how much each heartbeat interval differs from the next. It mainly reflects vagal activity. More about RMSSD , which only compares neighboring beats, SDNN counts every source of variation in the recording, fast and slow. Both branches of the autonomic nervous system autonomic nervous system Autonomic nervous system The part of the nervous system that runs the body without conscious control, including heart rate. HRV mostly reflects how it adjusts the heart from moment to moment. More about Autonomic nervous system feed into it. In a short recording at rest, most of the variation comes from breathing-linked changes driven by the vagus nerve vagus nerve Vagus nerve The main nerve of the parasympathetic nervous system, running from the brainstem to the heart, lungs and gut. Its fast signals to the heart drive most beat-to-beat HRV. More about Vagus nerve . Over 24 hours, slower rhythms contribute more, and 24-hour SDNN is the version used in medical research on heart risk.

SDNN grows as the recording gets longer, so the Task Force standards say SDNN values from recordings of different lengths should not be compared. A 1-minute SDNN from a watch, a 5-minute value from a chest strap and a 24-hour value from a study are three different quantities that share a name. They are also not interchangeable with RMSSD, so an SDNN reading cannot be checked against an RMSSD chart.

The lesson RMSSD, SDNN and lnRMSSD explained compares the metrics, and Why your devices disagree shows how the choice of metric changes the number you see.