The problem is that on at least some computers, "Sleep" defaults to "Wake on LAN" mode. This will awaken your computer from sleep in a very brief time if it is hooked to a router or directly to a cable or DSL modem.
To disable this, go Control Panel/System/Device Manager. Find your network adapter in the list and left-click on it. Then right-click on the network card (usually there is only one listed). Click on the "Power Management" tab at the top of the page, and then uncheck the box for "Allow this device to wake the computer." That should solve the problem.
It will still awaken on mouse movement. If you are using a wired mouse, it only has to move a VERY small amount to wake up the computer, like if the table is bumped or a printer is sitting on it and begins to stir. Wireless mice seem to require a bit more movement.
You can turn this off in a similar manner to that you used to turn the feature off on the LAN card.