Google is your friend.
Assuming you have set "put the computer to sleep if it is inactive for" to Never (according to
http://www.xvsxp.com/system/power_management.php - I don't do Apple) and there isn't something in the Schedule which is forcing it to, then you should check if autologout is disabled in "System Preferences -> Security -> Log out after X minutes of inactivity". (according to
http://forums.macosxhints.com/archiv...p/t-68164.html)