I often leave my Mac running overnight, and I sleep in the same room.
I kept noticing something that annoyed me which was the fans would often start whirring at full pelt just as I was going to sleep. The strange thing is that my MacBook (especially the new one) rarely makes a sound during the day. After a while they would spin down.
Last night the penny dropped – it’s the screen saver. The fancy, 3D hardware using screensaver. The moment I woke up the computer, the fan RRP started to reduce. So I promptly set my ‘Display Sleep’ time to 1min, and went to sleep soundly. Before, it was set to 15min – so it would eventually spin down (hence my previous observation as well).
Moral of the story: if you want your computer to stay quit right when you need it to be, use the ‘Display Sleep’ feature without screensavers (as attractive as they may be).