HRV is an indicator of how much stress you are experiencing. The variability increases when you are more relaxed and under less stress and decreases as your stress level increases. The relationship is a bit counterintuitive but it's a thing.

Good news; I'm gifting you with the fruits of AI, that has collated 200+ papers on the topic. I know, I know; this is generous.

https://github.com/catigula/recovery