Which is odd, because I haven't recently changed my mouse, and more confusing still: The monitor does go to sleep just fine. I've checked to see what devices are able to wake up my machine, but it only appears to be my mouse:
That's since been disabled, so it's definitely not that: I was having issues a while ago, but it was with my network adapter. (I can still manually ask it to go to sleep without any issue.) Windows 8 is set to sleep after 30 mins, and it used to work, but recently it's started refusing to sleep.
