by doing: powercfg -lastwake on the command prompt, you can see what is the last device/program/service that wakes up the computer.
Probably the system wants to do its maintenance cycle. I recommend to disable sleep, place the device in a cool place, open the Action center of Windows 7/8 (Search for action center on the search box/Cortana), and select "Security & Maintenance", expand Maintenance section, and click on Start and let it run. There is no progress bar sadly, so leave it overnight.
Once done, restart your system, re-enable sleep, and see if it helps