> I’m roughly around 40bpm at the moment, down from ~60bpm 10 years ago

To put that into perspective for other readers: I've been running for about 10 years also, but typically 2x a week (10k mid-week, 15~20k weekend), I have no real data on my heart rate from when I started, but at rest I'm now typically at 60bpm.

I measure almost daily due to medication and having a minor heart defect, and I have noticed that if I skip a week of running, it'll slowly go up, averaging at ~62bpm, but when I train for a (half) marathon I typically increase my distance a bit and try to train every other day (~3x/week) then my heart rate a rest goes down a bit to be consistently below 60bpm (58bpm avg).

40bpm is very low, for non-athletes this would be considered dangerously low, but I guess daily running at OPs distance would classify OP as an athlete. Also keep in mind that heart rate differs per person, some people just naturally have a low heartbeat.

He might be referring to his sleeping heart rate. Mine's around 40bpm too, and although I'm in shape, I am definitely not an athlete. There is a correlation with height (taller -> slower bpm). If you are young and fit, I don't think you have much to worry about a slow resting heart rate, in the absence of other symptoms.

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.