For a number of reasons that I’m too lazy to list here, I’ve setup macOS to lock the screen (and require my password) should I’ve been inactive for a couple of minutes. This usually works well, but sometimes it just… doesn’t.

Up until now, I never really bothered figuring out why; I usually lock my computer manually before leaving it, so it’s not very often that I experience the above scenario.

Well, yesterday I forgot to lock my computer during lunch, and when I can back to it, the screen was on and the mail app was waiting for me to complete a draft I’d been working on. Not good.

A quick search on the interwebs told me that there are a couple of ways you can troubleshoot this.

  • Using Activity Check out the Energy tab, and the Preventing Sleep column - if there’s something preventing macOS from sleeping, it should be listed here. You could also tell the app to show this column in the CPU tab, should you feel like it
  • Using pmset in your favourite shell. Just invoke pmset -g assertions, and you should get a textual representation telling you why macOS thinks it shouldn’t sleep right now

The culprit yesterday? Solution? Restarting the app, and tada - it no longer prevents macOS from sleeping.

Case closed.