Heart Rate Variability (HRV)

HRV is a measure of the time variability between the heartbeats. As an example if we have a resting heart rate (HR) of 60 beats per minute, it would be an assumption that the heart is beating every second. But in reality there will be difference between each beat in the order of milliseconds. so one beat might occur 997ms after the last, then next might be in 1003 milliseconds.