networking - Computer wakes by itself, but wake source is unknown

I have an interesting problem. I am using Windows 8.1 on my PC and I have WOL enabled from my BIOS and my LAN card settings and it works just fine.

But, since a couple of weeks back, my computer turns on by itself in the middle of the night.

Two nights ago, after it woke me up because of the noise, I removed the LAN cord and everything was well until the next morning when I powered the PC myself. I also thought it could be a peripheral, so from Device Manager I went through every device there (apart from the LAN card, which I need because of the WOL), disabling Allow device to wake the computer from "Power Management". Also, in the LAN settings, I checked and the network card can be wake the PC up just by using Magic Packet or Pattern Mach.

The problem persisted last night (it woke around 5am).

I started reading forums and saw the idea of looking what last turned on my PC, so I checked with powercfg -lastwake and it returned the following:

Wake History Count - 1
Wake History [0]
 Wake Source Count - 0

Because of the answer, I couldn't find a thing, therefore I checked the Windows logs, filtering them by Power Issues. I found the last wake my PC experienced, and the log was as it follows:

The system has returned from a low power state.
Sleep Time: ‎2014‎-‎05‎-‎08T23:38:33.848063300Z
Wake Time: ‎2014‎-‎05‎-‎09T01:56:48.134397800Z
    Wake Source: Unknown

This is the reason I have posted the issue here, even though there are many posts about this. I didn't want to repost, but I never found this same problem (with wake source unknown) in any post or forum.

I hope someone has an idea of what may be the problem.

Answer

Disable Windows 8 Automatic Maintenance.

By default, it will wake up your computer around midnight and run some optimization and cleanup tasks.

For reference see What is the "Automatic Maintenance"?