What is the association between low HRV and mortality risk?

What does high and low HRV mean and why do athletes strive for a high HRV?

What factors that can raise or lower HRV?

HRV, health and longevity

Can you do a deep dive on HRV and how you use it to gauge fitness, health etc? What do you do if anything to improve or impact your HRV? Do you believe in neurostimulators (like Apollo) for this (they have trial data)?

How and when should we measure HRV, and what are the best wearables?