SDNN

SDNN (Standard Deviation of NN intervals) is a time-domain HRV measure capturing the overall variability of normal heartbeats. Per HRV Task Force standards, SDNN is primarily reported over 24-hour recordings, where it reflects both sympathetic and parasympathetic influences along with circadian rhythms and longer-term low-frequency variability that contributes to 24-h SDNN. Short-term 5-min SDNN is heavily influenced by respiratory/vagal variability, though RMSSD and HF power are more specific markers of vagal modulation. Low 24-hour SDNN is associated with increased cardiovascular and all-cause mortality risk, particularly post-infarction.