I have an iMac as well, a 2007, love it. I did have a problem with 'sleeping' it some time ago when I tried using a different mouse. I can't even remember which one it was. But it did the same thing. I do not like Apple's mouse so I use a little notebook wireless mouse by Logitech and I have not had the same issue again.
Sorry I wish I had a direct answer. But it could very well be something you are using via USB.
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