MacWorld has a brief tutorial on how to “Turn your Mac into a musical alarm clock”. Aside from being a handy little workflow, it also illustrates how to schedule them to run when a sleeping Mac wakes up:
“Save this workflow as an application, open the Accounts preference pane in System Preferences, click the Login Items tab and drag your workflow to the login items list.
Open the Energy Saver preference pane, click the Schedule button, and choose the time to wake up your Mac. When it wakes up, the Automator workflow will run, telling your Mac to play the selected iTunes playlist.”