It’s measuring your blood oxygen concentration. Red light gets shot into your arm and the light that gets reflected back to the sensors, tells the watch what your O2 concentration is.

Probably it is measuring blood oxygen saturation

WatchOS 10 comes with its very own Hell Portal.

We think you’re gonna love it.

https://preview.redd.it/v7ze2viejxxb1.jpeg?width=3024&format=pjpg&auto=webp&s=7206ab5be24db5e12fc0515bc1feeb04b42c8b95

Me, trying to sleep