Sleep mode is the old XP and 2000 era "Standby". It turns your hard drive, monitor, case fans, other drives, and any USB devices to very low power mode. It puts your processor to a very low power state which creates very little heat. I agree that sleep is for greenies, but if you are away for more than an hour but not for many hours, sleep mode is good. It rests your hard drive and puts your processor in a low power mode so almost no heat is created. I would recommend sleep mode.