I have done regular exercise through most of my teen and adult life, and still cycle regularly.
I have never got my resting heart rate below 60 (although tbf probably never measured it before my 40s), and currently it seems to hover around mid 60s. I have no idea of my heart rate when sleeping and only measure it infrequently.
I firmly believe that, other than the occasional outrider, genetics is the main driver of heart rate.
Agreed, to a degree, i e. Yes, genes are, of course, a key factor too.
However, the medical bods seem to think what would otherwise be a "worryingly" low resting heart rate (certainly c low 40's, ie bradycardia) is ok if you regularly do cardio exercise.
Does seem to make total sense, in terms of training and improving overall fitness of the heart, therefore necessitating less beats per minute for a given scenario.
Maybe your resting heart rate would have been higher than mid 60s if you hadn't been so fit and active, and mine higher than low 40's if I hadn't? Perhaps that was your genetic mix alongside being fit.
My wife's uncle, was a keen amateur footballer in his 20's and 30's, and ran a decent time in the London marathon in his 40's, but he says his resting heartrate has always been in the mid 60's too.
No right or wrong, just how it is I suppose.