What is SDNN?
SDNN is the standard deviation of the intervals between normal heartbeats in a recording. Where RMSSD isolates beat-to-beat changes, SDNN describes the overall spread of the intervals, so it is shaped by every rhythm in the recording window — including slower ones — and depends strongly on how long the recording is.
Apple Health has long recorded heart rate variability as SDNN, so Apple Watch HRV values should be compared with SDNN norms, not RMSSD norms. Like any HRV measure, a single SDNN value is not a diagnosis; trends against your own baseline under comparable conditions say more.
Read the science → the full ONDA Science page on SDNN is in preparation; meanwhile see RMSSD on ONDA Science, which explains how the two metrics differ, and why Apple Watch HRV can look low.