I know I have a PowerMac, and this is an iMac thread, but it was such an interesting question i had to join in. Actually, i will mention my old iMac so its not completely cheating!
With my iMac G3-Well, i left it on. Despite having a lot of RAM, it still booted up really slowly. Didn't feel like waiting. When it's sleeping that light that pulses gets REALLY annoying, since im in sleep mode as well (ha), and i'm the type of person who has trouble sleeping and needs everything off/complete darkness. So i would just cover the light with a folder or something.
Now with my PowerMac G4 its quicker, so i turned it off at first. Now i leave it on because it's losing its speed lately
, and it's set up so that the tower is being blocked by my desk on the side, so i don't really see the light that much.
Also my PowerMac is my alarm clock as well. One out of three. I downloaded the MP3 alarm clock app, and turn my speakers up. Then i set my cell phone to the most ANNOYING ringtone (when the sains to marching in on an LG phone is horrible) and turn it all the way up and put it by the door, then my regular alarm clock by my bed. That way i HAVE to get up to turn off all the madness, and when im up, im up.