SDNN (Standard Deviation of NN Intervals)

SDNN stands for Standard Deviation of NN (Normal-to-Normal) Intervals and is a fundamental time-domain measure of heart rate variability. It represents the overall variability of the time intervals between consecutive normal heartbeats over a given recording period. SDNN reflects the combined influence of both sympathetic and parasympathetic branches of the autonomic nervous system.

SDNN values are highly dependent on the recording duration, making it important to compare measurements taken over the same time period. For short-term recordings (5 minutes), normal SDNN values typically range from 50-100 ms in healthy adults, though values vary significantly with age, fitness level, and health status. Low SDNN has been associated with increased cardiovascular risk and all-cause mortality in clinical studies.

Contactless measurement of SDNN using rPPG technology enables non-invasive assessment of overall autonomic function. While SDNN from short rPPG recordings provides a snapshot rather than the comprehensive 24-hour analysis used in clinical cardiology, it offers practical value for wellness monitoring, fitness assessment, and identifying individuals who may benefit from further cardiovascular evaluation.