sleep mode is for greenies and realy doesn't harm the hardware it is faster to get back to work than a full off/boot situation but if you realy worried about it just put the monitor into sleep mode and leave the rest running full noise
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.