Resting heart rate seems to be at least a measure of health in some way, but could contribute to longevity as well. I’ve had medical friends suggest that there is a limit to the number of beats your heart can beat in its lifetime.
What I’ve noticed from my fitbit is that if I drink alcohol, I will have higher resting heart rate for a couple days. Same is true if I eat high carbohydrate meals. I’m insulin dependent diabetic and have noticed that if my blood sugar goes slightly low in my sleep, I will wake up and my fitbit will show an elevated heart rate.
To an extent, this is all just telling me what I already know: Being more fit is good. But resting heart rate can also serve as an second marker to other measurements. Thinking about the mechanisms could provide insights or creative theroes to explore in other ways. Why does heart rate go up when sugar drops below ideal? Probably because your cells aren’t getting their expected glucose so they kick up heart rate to compensate. Why does hear rate get very elevated if you have very high glucose? My guess is that high glucose caused by low insulin is similar to low glucose in that cells can’t get glucose into them.