This is a weird post. I'm by no means an expert but I've read some papers on HRV and this is extremely well known in HRV research. So much so that these facts (

This is a weird post. I'm by no means an expert but I've read some papers on HRV and this is extremely well known in HRV research. So much so that these facts (and others) are basically copypasta at the beginning of a lot of papers. People have definitely quantified this before, like a lot. Also, the general advice is that it isn't expected to be "accurate" and the measurements are relative anyway (both to the individual and to the standard of measurement.) There weren't a bunch of researchers (or even biohackers) walking around who thought SDNN or RMSSD could be substituted for EKG.