When I finish my work, I set up my Windows 8.1 PC to sleep, but had found that it started to wake up at random times, and sometimes it stayed on for long, other times, it went back to sleep after a few minutes.
I realized some device with power management abilities should be the culprit, so I went to System, clicked on Device Manager and desabled the network card and the mouse to wake up the PC.
That solved the problem, untill I decided to install the Intellipoint software for my Logitech mouse, and the PC started waking up at random again. I checked back on the mouse power management settings and, yeah, it seems just intalling that application gave back the mouse ability to wake up the PC. So I disabled it back and now is problem solved again.