Possible Duplicate:

How can I shutdown my PC when the system is idle?

In Ubuntu 11.10, when I go to System Settings > Power, I get the option to suspend my machine after it's been idle for a set period.

How can I set the machine to shut down instead?

To be clear, I am not interested in setting the machine to simply shutdown after a specific amount of time or at a specified time, as is possible with the shutdown command in terminal. I am interested in my machine automically shuting down and powering off a set period of time after I last did anything.