I know because I’ve compared the numbers with my polar chest strap for everything and it’s all spot on for resting and sleeping. I’ve also been taking hrv measurements for a couple of years with some well known hrv apps and the polar strap and the hrv readings seem consistent to me.
Also compared to how I’ve felt day by day and also that the whoop sensor is an hrv sensor and has been used in many studies.
I’ve no reason not to trust it.
Also forgot to mention in my last post about a guy on YouTube called the ‘quantified scientist’ who does in depth analysis and testing on all sorts of health tech including the whoop. Worth checking out.