As of March 6, 2007, Nightscrawlers Chat users who have been idle for 50-60 minutes will be automatically logged out. Even though you may not be viewing and replying to messages, your browser is still constantly requesting and displaying user information, room information, and new entries. If you are not actively using Chat, this is a waste of bandwidth.
This change should not impact you as long as you are not idle for hours at a time. Private messages also count as activity, for the record, so you will not be removed from the room because you're privately conversing with another user and not talking in the main room.