Apparently, HRV recorded in a standing position may be more sensitive. Not sure I qualify as a “well-trained athlete”, but could be worth trying 
Couldn’t hurt. Not sure about how much one should take away from a study with n=1 though…
My yearly HRV analysis confirms better sensivity for standing measure. Also i’ve found that Oura ring nightly rmssd and polar h10 morning standing rmssd are both correlated with physical load. I’m using h10 with a little water instead of a gel and it provides enough accuracy to detect these effects.
2 Likes
My watch (Garmin Fenix 6S) automatically records an “overnight average” HRV, so I was wondering if that might be useful, and how those values compare to manually recorded measurements.
I used the HRV4Training app for 2½ months to take 1-minute readings shortly after getting up every morning, while standing, using my phone’s built-in camera (Pixel 6a). I repeated any readings that didn’t show as having “excellent” signal quality (washing my hands with hot water seemed to help).
Then I used OpenAI’s Code Interpreter to analyze the data.
I was slightly above average active during this time, but the two vertical lines were the only two days I “overreached” a bit (according to Garmin, and subjectively).
Results: There is no overall correlation between the two measurements (except maybe for a week or so). Garmin’s values appear to be a bit less random and are trending upwards, plus don’t require any effort to collect, so I think I’ll just go with those 
I now suspect that most of the temporary weight loss I see after prolonged exercise is due to depletion of glycogen: We can store about 0.5kg of glycogen, and since each 1g of glycogen is stored with 3g of water, that adds up to about 2kg. It’s not inconceivable to use up half of that, or more.
Weight lost due to dehydration is recovered much faster, and weight lost due to burned muscle/fat is going to be much less, and require much longer to regain.