What OS are you using? Windows 7 has stuff scheduled to run automatically, like disk defragmentation and updates. Have a look at Task Scheduler and see if there are things there scheduled to run in the middle of the night.
Also, if you open a command prompt to run as administrator and type in powercfg - lastwake it might tell you what woke your computer (mine always says it doesn't know, but theoretically it should tell you) or if you put in powercfg -waketimers it should tell you what is scheduled to wake your computer up.
I'm no computer expert, but try googling info about the task scheduler or the powercfg command and it might help you find out what's going on.
Hi. I'm sure you all get this question often, but how do I stop my computer from turning on by itself at night? I have built my own computer, and recently it began turning itself on around two in the morning. I have checked the BIOS for any "wake on" settings, and they are all off. Any help would be greatly appreciated!