Heart Rate Variability

HRV is simply a measure of the variation in time between each heartbeat. This variation is controlled by a primitive part of the nervous system called the autonomic nervous system (ANS). It works regardless of our desire and regulates, among other things, our heart rate, blood pressure, breathing, and digestion. The ANS is subdivided into two large components, the sympathetic and the parasympathetic nervous system, also known as the fight-or-flight mechanism and the relaxation response.

Standard Deviation of NN intervals (SDNN) is a measure of Heart Rate Variability (HRV). NN intervals refer to beat-to-beat intervals. It is the time interval between each heartbeat. SDNN is the standard deviation of the time intervals between each heartbeat, expressed in milliseconds (ms).