I’ve been able to track my sleep using my Apple Watch since upgrading to the 7, but I never used it for that purpose because I always needed to charge it by bedtime, and that made the most sense to me.

Upgrading this past November (my birthday) to an Apple Watch X, I was able to benefit from longer battery life, yes, but also swapping watches when the juice in one gets low. Especially since the whole Masimo thing, it’s nice to be able to still measure my blood oxygen. So long story short: I have plenty of runway to track my sleep now.

Apple Health is fairly neutral on the subject of your sleep quality. It reports your total sleep time, and separates your sleep into Core, Deep, REM, and Awake categories.

Apps like Sleep+ and the Withings app offer more qualitative assessments of your sleep. Neither like my not-quite-seven hours of nightly sleep. And they both report that my sleep isn’t restorative, because there’s not enough REM or Deep.

I figured that’s probably due to the length of my sleep. If I slept longer, I’d get more REM and Deep, right?

Not so fast.

Friday into Saturday often yields more sack time, as does Saturday into Sunday. You might expect the 11 hours total I got last night would have yielded some extra REMs. But that’s not the case.

There are a few hypotheses you could generate to explain this. First, it’s possible that your REM and Deep sleep depend on your average nightly rest, not single sessions. That wouldn’t surprise me. It’s also possible there’s something I could be doing differently to influence those numbers. There’s always the possibility that you can’t do much about your sleep phases, which one of the apps even mentions.