RMSSD

RMSSD (Root Mean Square of Successive Differences) is a time-domain HRV metric defined as the square root of the mean of the squared successive differences between adjacent NN (or RR) intervals, expressed in milliseconds. It is considered the most reliable short-term marker of parasympathetic (vagal) activity and is widely used in wearables and rPPG-based readings. In longevity research, RMSSD is tracked as a daily indicator of recovery, training adaptation, sleep quality, and acute stress load.