Was just coming to say that there's definitely a genetic component. Even in my more sedentary periods of life, my resting HR never really peaked above ~55 BPM. Playing tennis and going for regular runs with my husky puts my resting HR at 38-40bpm. My father who also runs and plays sports has a similar heart rate.

Just so we're clear on definitions, resting HR is your heart-rate AT REST (meaning sitting or lying down quietly), it's not necessarily your HR while you are sleeping which may be potentially lower.