I asked my cardiologist about it and he said "adaptation". Apparently you start using oxygen more efficiently (mitochondrion population hike?) so your heart doesn't need to pump as hard.
If that's the case, isn't it amazing??
If you think about most fitness adaptations (e.g. reaction to weight-lifting) it takes days to get any benefit, often weeks. And this is happening while we're still riding 18hour days!??
I have looked into this in the past, and there seems to be verrrrry little hard research. I think sports science is focused on elite sport and durations upto Ironman - multi-day stuff probably just doesn't pay!
I had a gander at the mitochondrial life cycle and learnt very little other that they arise from existing mitochondria.
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That ^^^ is a bit like coming into the film halfway through, since a lot of the characters are proteins we haven't met yet, and to be quite honest I'm not that curious. (I note that parkin is involved, which might suggest that Northerners make good Audaxers.)
Anyway, all the abstracts I glanced at involved studies over 10 weeks or so and none about short-term evolution.
Upon reflection, I'm now thinking that decreased heart rate mostly results from fat metabolism kicking in, which might also be what the cardio called adaptation. Might also explain why I used to fly on the third day and zonk on the fourth - inadequate fat reserves. Unfortunately, that is no longer the case.