I have researched this but cannot find this particular issue with Time Machine and sleep mode. In the past I only used time machine to back up my system only when there were major changes, new software, etc. A few days ago I turned it on and left it on and now my iMac will immediately (within a second) wake back up when I put it to sleep even though Time Machine is not in operation. That is,TM is on but not performing any backups. I understand it will not go to sleep if a backup is in operation, but shouldnt it sleep if one is not? If this is not normal, how do I correct it? BTW, I have a mid 2011 iMac 27 with intel i7, 8Gb memory, running SL 10.6.8 with backups to a WD MyBook 500 Gb USB external drive.