One feature a number of fitness trackers offer is a metric called “heart rate variability,” or HRV, which is the interval of time between each heartbeat. But given that we tend to think of our hearts beating at a steady and predictable pace, the fact that the interval between each beat varies can be a hard concept to grasp.
However, since HRV is responsive to stress—whether it’s anxiety, sleep deprivation, overtraining, or any of many other physical and mental stressors—keeping an eye on your HRV can be a way to gauge whether your overall lifestyle needs a change. For example, a lot of professional and endurance athletes will keep an eye on their HRV as a way of gauging how their overall stress levels may be affecting their performance.
See What Your 'Heart Rate Variability' Says About Your Health
Our heart rate variability can change, and that's a good thing.