It is most likely seeing network activity which causes it wake. You can change this by going to Network and Sharing Center, click on Change adapter settings, right-click on the adapter you want to change and select Properties. Clik the Configure… button then the Power Management tab and clear the checkbox for ‘Allow this device to wake the computer’. Click on OK.
Some times this happens when there are background tasks that are set to be run at specific times. Have you checked your task scheduler to check if there are any background tasks set there? These tasks are not necessarily set by windows, they can be set by third party programs.
Another possibly In control panel -> administrative tools -> Task Scheduler … find under Windows -> media center … find mcupdate_scheduled in the history window. Click on the ‘triggers’ tab and then click on ‘properties’ in the actions window and DISABLE the automatic update.
Another Prevent a Mouse from Waking Windows 7 up from Sleep Mode
Give those a try and let us know