I have my iMac set to Display Sleep - 15 minutes and Computer Sleep - 15 minutes. Whenever the computer is left alone for 15 minutes, just the display goes to sleep, not the computer. I never had this problem until about a year ago. I have the computer set to wake for network access, and I currently have file sharing enabled with a Windows computer. I turned off file sharing to experiment, but the computer still won't go to sleep (just the display). What do you think is going on? Could it be a software glitch? I already saw the page on Apple's site that explains why your computer may not go to sleep, but I don't think it's any of those things because it was working fine about a year ago. I have a feeling it is a software glitch that probably happened after a new software update to the OS or something, but I don't know.