I ran this command just now after I came home to my computer on to see what woke it up:
C:\Windows\system32>powercfg -lastwake
Wake History Count - 1
Wake History [0]
Wake Source Count - 1
Wake Source [0]
Type: Device
Instance Path: PCI\VEN_8086&DEV_8CB1&SUBSYS_8CB11849&REV_00\3&11583659&0&A0
Friendly Name:
Description: Intel(R) USB 3.0 eXtensible Host Controller
Manufacturer: Intel(R) Corporation
This implies a USB device woke the computer up even though I obviously didn't touch anything?
Do you use a battery-backup UPS system, like APC or TrippLite, connected to a USB (possibly USB3?) port?
Check the properties in the Device Manager for the UPS. If the checkbox for "Allow this device to wake the computer" is checked -- uncheck it. If this would leave you vulnerable during a power outage while the computer is asleep, make the system hibernate an hour after it goes to sleep to minimize the window of time when that might happen.
Sometimes, if there is some minor power-glitch that forces the UPS from AC-line to battery for a few seconds, I think the UPS may attempt to wake the computer to shut it down according to the settings in the UPS software.
After that, you might want to tweak the USB nodes in Device Manager for certain parameters that might cause this. I cannot remember them precisely offhand, but I was having trouble with a USB-connected XBOX-360 game-controller, found some indications on other forums, made some tweaks, and that problem "went away." In fact, the tweaks you may want to make might be found in the device-node itself, rather than the USB section.