lts is there a way to enable sleep mode and wake on lan?

I was wondering if it were posible to make my server go to sleep after a set period of time, but still be listening for lan requests. I use my server as a media server that might get used 3 or 4 hours a day and it is really a waste of power to have it running all the time. However I don't want to run up to the second floor to switch it on when we want to watch a movie. I saw a few posts about stopping this from happening but how do you enable it?