My Windows 8 PC had taken to waking up by itself, after dozing for some hours. I checked for obvious causes - network card being allowed to wake the computer when sent a magic packet, keyboard or mouse being allowed to wake it, scheduled tasks going off (such as Windows Media Centre), media sharing across the network. None of these was relevant.
I did notice in Event Viewer that the computer would always wake exactly 3 hours after going to sleep. What had I asked my computer to do after 3 hours of inactivity? Hibernate! Yes, I'd set my computer to go to sleep after 20 minutes and hibernate after 3 hours. For some reason it was unable to transition from sleep to hibernation and just remained awake instead. The computer is capable of hibernating from being awake, so it didn't initially seem likely to be the cause. However, since I already use hybrid sleep there wasn't much gained from using hibernate so I've turned it off. Since this modification my computer has happily enjoyed uninterrupted sleeps.