As you know, I’ve been using both an iPhone recorder app to record elements of my dreaming, as well as another iPhone app that records whether I’m snoring etc. I have an Apple Watch Series 4, which is several years old but now because of a new software update that came yesterday (Watch OS9) it now tracks sleep cycles. And today I compared what it recorded with the various time stamped recordings that I have of my dreaming and snoring last night and there’s a really good match. I’m very impressed. Here is a YouTube channel that I used to follow that has come to a somewhat similar conclusion, about the accuracy of what it tracks .