Heart Rate Variability (HRV)
Heart Rate Variability, or HRV, is considered by scientists and physicians to be an excellent non-invasive measurement of nervous system activity and heart health.
HRV can be calculated from basic pulse rate data. It refers to the tiny beat-to-beat variations in your heart rate. For example, the time between one beat and the next may be 1 second; the time from the 2nd beat to the 3rd beat may be .6 seconds.
The variation in time from one beat to the next is called HRV. The time between beats is called the inter-beat-interval (IBI). There are different ways to analyze the variations, but in its raw form, HRV is calculated by taking the standard deviation of the differences in the IBI. These differences are called SDNN, which stands for Standard Deviation of Normal to Normal intervals.
A great deal of research has shown that high HRV (i.e., high SDNN) is associated with better physical and mental health (see References).