Do you have a bluetooth keyboard and/or mouse? My older white iMac did the same thing until I disabled the "wake on bluetooth" option (sorry I can't get more exact - I'm at work and my edu. model iMac is "tooth-less."). The big downside, of course, is that you can't hit a key or click to wake it up (unless one is wired, of course) - I have to press the power button on mine, since I have both BT KB and mouse.