Heart rate variability is the time interval between your heartbeats. This interval is controlled by your autonomic nervous system. HRV is an important metric to use if you know how to find it. However, its tricky to measure, not like heart rate or pulse.
The autonomic nervous system is your body's stress or nervous system regulator and contains two main parts: parasympathetic (deactivating) and sympathetic (activating). Our heart rate variability lets us know how easily and quickly we can return to a calm, resting state, after responding to stress. In other words, how physiologically and mentally resilient and adaptable we are to our ever changing external environment. HRV an important indicator of present physical and mental well-being and a statistically useful forecaster of our future health.
The good news is we can improve Heart Rate Variability (see below) and we can now monitor HRV without expensive medical testing, by using new consumer technologies, like wearables and phone apps that are designed to measure HRV.