Computer Shuts Down On Its Own At Random Intervals?

1. computers shuts down on its own2. The shutdown is elegant, all programs close down quickly and there is no BSOD or anything2b. it gives me the impression that the PC is acting as if someone ordered a shutdown with the -force option, closing all apps by brute force.3. It almost always happens very late at night between 2 to 5 am.. I have yet to realize if there is a pattern to this whole thing. It seems random and happens 3-4 times a month5. When i open the PC everything works like a charm.6. I have scanned the PC with Microsoft security essentials, Avira antivir and malware bytes antimalware, and no sort of virus was found.