Can I schedule Time Machine to Back up only at certain times?

I have a MacBook Pro with Snow Leopard. I do not have a mobile me account. I use Time Machine to back up to an external drive. The problem is that Time Machine is constantly backing up (or at least it seems that way) and when it does, it slows down my computer to the point where I have to tell it to stop backing up. I looked and Googled, but couldn't find an answer to my question. Is there somewhere I can tell my Time Machine to only back up at like 3:00 am when I'm not using my computer? Thanks.