Agreed. Also for athletes, not sure its very helpful. People obsess over this stuff. When I was living and breathing powerlifting, going into a workout knowing I wasn't at 100% recovery would mess with my performance. And if it was a PR attempt day, forget about it. Its the opposite of placebo. Its a nocebo. But when you aren't focused on how optimized you are, I've hit numerous PRs on days I was hungover or hadn't slept- days the whoop would have told me to take it easy. We are more complex than machines. But from a strictly longevity based standpoint, they are probably useful.