You can try these steps and see if it works:
a. Open Device Manager by clicking the Start button, clicking Control Panel, clicking System and Security, and then, under System, clicking Device Manager.
If you're prompted for an administrator password or confirmation, type the password or provide confirmation.
b. In the list of hardware categories, find the device that you want to allow for waking your computer, and then double-click the device name.
c. Click the Power Management tab and then click the “Allow this device to wake the computer” check box.
d. Click OK.
If that doesn't work, check your installed windows updates to see if the following update is installed. If not, try installing it. Apparently it has corrected sleep issues on many systems.
Windows Update KB2955164
http://www.microsoft.com/en-us/download/details.aspx?id=42767