Is there a way to keep iChat and Adium from logging me out when my computer sleeps?

When I’m logged into Adium or iChat and the computer is in use everything is fine. When I wander away or go to get something to eat and the computer isn’t being used, my computer goes to sleep after about 10 of 15 minutes. Now, when my computer goes to sleep, I automatically get logged out of Adium or iChat and all my conversations are ended. Is there a way to stop this from happening so that when I wake my computer up it’s as if nothing has changed?
If you need to know, I’m using a MacBook Pro.
I’d rather not never put it to sleep because I don’t want to ruin my battery in any way.