It’s linked to your autonomous nervous system. There is your sympathetic system that lowers your HRV. This happens when you do exercice, when you are more active or stressed. And your parasympathetic system, that allows your HRV to go higher. This happens when you sleep, digest and other stuff. You can look this up. But in short, the more you move, the lower your HRV (b/c of your sympathetic nervous system = SNS), the more you relax, the higher your HRV (because of your parasympathetic nervous system = PNS). Hope this helps a little.