Why does my mac logout on its own?

Hey guys maybe you guys can help me with this. My powerbook g4 seems to log out on its own at random points. Its never while i am using it but like if i leave it on over night it seems to log me out. It has been doing this since i got it so i think it might be a feature somewhere. also i am not getting any errors when i log back it like it restarted but just logs me out. This is quite a annoyance because i like to leave my aim and other programs running but when i come back my computer has been logged out. Please help!

Thanks

~Justin

There is an auto-logic setting somewhere in System Preferences. I can't be more specific (I'm at work and being forced to use XP) but I know it's in there somewhere ...

sweet man its in the security tab of sys prefs. i dont know why i over looked that but thanks man!