Apple Watch uses SDNN to measure heart rate variability, a metric historically used medically to assess autonomic variation and parasympathetic function over 24 hours.

“now the only one I'd say that's different is Apple watch actually they use what's called sdnn which is just the standard deviation of the B2B intervals why they do that uh I'm not sure that's one it's historically been used medically and they'll usually measure it for 24 hours and they'll just kind of look at like do you have any autonomic variation does the paretic system function well at all and it's kind of a gross measure it's not nearly as nuanced because we're not measuring Valle input at”