If Windows 8 is trying and failing to update a program marked important, you can forget about sleep working properly. There will be a process running called Windows update and it will both prevent sleep from occurring AND wake the damn machine up. You can kill the process but it will just start running again!
Solution? HIDE THE UPDATE that is bombing. Right click on the failing update and click on "hide update". That will fix the issue.
Mine was on OneNote which I never use. If it seems like an important program, I guess you just have to uninstall and reinstall and keep your fingers crossed. I tried to "fix" the program for a bit and ran error checking and scannow but it did not fix it so I am going to just ignore it.
Now I can go back to sleep...