If you've ever woken up feeling sluggish, tired...and confused because you've got an excellent sleep score after clearly a bad night's sleep, you're not the only one. Not every tracker records heart rate variability (HRV), yet it's one of the most interesting metrics on a fitness tracker.
HRV is the tiny difference in time between each heartbeat. The nervous system, which is made up of the sympathetic branch (fight or flight) and parasympathetic (rest and digest), controls it.
