I have moved to Win10 and everything was working fine for a couple of weeks, up until a few days ago - my computer stopped sleeping reliably. There were a handful of things that happened around the time the problem cropped back up, including several Windows Updates. The one difference with Win10 vs Win8.1 and this blasted problem, though, is I had the problem from a fresh install with Win8.1 - not so with Win10.
I'm currently working on a hypothesis that looks promising. It appears Adobe Acrobat Reader DC (version 2015.009.20069) may have been the culprit for me, though I need to do further testing to be sure.
With this thought, I wondered if for some strange reason Adobe Acrobat Reader might be involved with some of the sleep problems reported in this thread? Could someone still having issues in Win8.1 (and especially if they're using a MS account to log in, seeing as switching to a local account in Win8.1 is what solved my sleep issue in Win8.1) try uninstalling Adobe Acrobat Reader and do some testing to see if that solves their problem? I realize it's a long shot, but maybe? ...