Heart Rate Variability (HRV) is the physiological phenomenon of variation in the time interval between heart beats. It is measured by the variation in the beat to beat intervals. We call this the time domain. A healthy heart rate variability is reflected by lots of variation in the time domain between successive heart beats. This would indicate a very robust autonomic nervous system. Someone with less variation between one heart beat to the next has low heart rate variability and indicates decreased autonomic nervous system regulation. The importance of Heart Rate Variability is that it is controlled by the autonomic nervous system and therefore is a window into its function.