Something else is waking your PC and OneCare is simply detecting that it has access to the Internet, so it checks for updates. It always checks for updates immediately upon connection to the network.
OneCare will not wake a PC from sleep or hibernation for any action. In order to get OneCare to perform a Tune-up overnight, for example, you would need to create a scheduled task in the Windows Task Scheduler for another function (non-OneCare) to wake the PC shortly before your scheduled Tune-up is to start.
-steve