I haven't encountered this issue too much, but when I know that I am going to be away from my computer for an hour or more, I log off. Sure, I know that I may not get back on as soon as I return, but it would just be rude of me to not log off. Also, I can't sleep with my computer left on. The lights from the tower and mouse would prevent me from sleeping, not to mention how hot it makes my room.