Heart Rate Variability (HRV) is positively correlated with health outcomes, including brain function, body performance, and longevity.

“having a higher HRV or Heart Rate Variability is a good thing, right? Normally, if you hear something like Heart Rate Variability, it sounds like a bad thing. Turns out it's a great thing. Heart Rate Variability is essentially the distance or the time, rather, between beats of the heart. ... But actually, it's well-known to be correlated with a number of positive health outcomes, including things related to brain, and body, and longevity, and performance to have high Heart Rate Variability.”