Uzaren.com

Computer must be awake for scheduled shutdown to occur.

In Mac OS X 10.3 and later, 
you can schedule a time for your 
Mac to automatically 
shut down by setting a time 
and interval in the Energy Saver 
pane in System Preferences. 

However, please note that 
your computer must be awake 
at the time that it's scheduled 
to shut down in order to do so.
 
If your computer is sleeping at this time, 
it will continue sleeping instead of shutting down.