Stop iCal from Waking my iMac

I have all my appointments on my iMac that that syncs with my other iOS and computers. But when I am done using my iMac, I put it asleep. Frequently I find that when I return, the iMac has turned back on, wasting electricity. As far as I can tell, it is caused by an iCal event.

How can I prevent my iMac from waking up from an iCal event?

Please don't tell me to have the iMac default to going back asleep after a certain period of time. There are situations when I want my iMac to stay on, say when performing a task that can take hours. I don't want it to fall asleep during that event.