A feasible idea?

I've been reading posts and watching videos on interupts. I love the idea of being able to put Arduino into sleep mode to save energy, and have it wake ip when triggered. How is it possible to have it turn an led on for 30 seconds, turn it off, hhen go back to sleep? Thanks for any input!

Arduino Playground some info on sleeping.

A video tutorial
Arduino Power Saving Tutorial: Clock Pre-Scale and Sleep

Hope it helps