SDNN as measured by Apple Watch is a less nuanced measure of HRV compared to other methods because it combines rest and activity periods.

“it's kind of a gross measure it's not nearly as nuanced because we're not measuring Valle input at a particular time we're just measuring across longer periods of time and maybe that's why they did that it seems to me that that would also introduce a bit of noise because you're combining being at rest with being active yeah exactly and you would I mean I don't know if it's the right word but you'd be penalizing people for being more active because the more you exercise the more sympathetic tones you have during exercise the more you're crushing the variability correct yeah”