2011 27" iMac with the 3.4 processor. She does go to sleep on occasion, like when I manually tap the power button, but she has trouble staying asleep sometimes. I also believe that the 'auto sleep after X time' feature in Energy Saver isn't working quite right. My screen will go dark and then turn off, but she's not asleep as I can hear the computer humming quietly unlike the absolute silence during sleep mode. It's lead to quite a few annoying situations where I come back to my computer after a number of hours and find that it hasn't been sleeping since last I left it. I just don't want to be burning up so much electricity. Any one else experiencing this? Tips for solving issue?