Just a quick note to share the solution to a problem I had with Sleep mode after upgrading to Windows 8.1, namely, the computer would not go to Sleep automatically.
For me, the fix was to leave the Homegroup.
I did this by typing Homegroup into Start, selecting Homegroup Settings and clicking Leave.
Normally, when trying to solve Sleep-related problems, your best bet is to open an Administrator PowerShell or Command Window and run the command
However, in this case, powercfg always stated that Nothing was keeping my computer awake. Not terribly helpful.