My gps watch supports recording of this which I can later analyze with the free Kubios HRV software (or some non-free software like Firstbeat).
On a first test it seems like my HRV values are higher than the normal, and fit around the 25 percentile of 'sportsmen' group according to a study I found.
Athletes and fit, active people have higher values in general. The values go down in case of disease, overtraining, etc. The HRV is regulated by parasympatethic and sympatethic nervous systems, and various other factors as well:
I need to look better at it, but in any case it seems like a more powerful and reliable general health status metric than the Buteyko 'control pause' test, although they are probably related. From what I found online, immediate HRV can be somewhat controled and imroved by synchronizing proper relaxed breathing and meditation (which makes sense, as breathing has a large effect on the parasympathetic/sympathetic nervous systems).The main inputs are the sympathetic and the parasympathetic nervous system (PSNS) and humoral factors.
Factors that affect the input are the baroreflex, thermoregulation, hormones, sleep-wake cycle, meals, physical activity, and stress.
More reading (that I need to do as well, I'm kind of bookmarking here :))
http://www.iam-u.org/index.php/20-reaso ... you-can-do