I've had this problem for a few days now. When I enter sleep mode, my iMac will go into sleep mode. But the computer will keep running. (The screen fades black but that's it). Imho it's not just the fan, because when i power it on again the screen is there immediately (faster that its supposed to pop up). I don't know what to do, the only way i can stop my mac from running is by shutting him down. Which as you know is not all that good for a mac. I have an iMac 2011 base model and i'm running the latest version of lion. If anyone knows what to do, please tell me.