I have the same problem as well on one of my older iMacs running 10.7.5. It's clearly a bug. There's no setting in System Preferences / Energy Saver that you can adjust that will resolve the problem.
You are out of luck, unless you want to start over and reinstall IOS from scratch. Then, install all your apps from scratch and copy back all your documents, files etc. If you try starting over, don't forget to cross you fingers, because it might be one of the apps that exposed the bug in IOS.
I "solved" the problem by installing the app Caffeine and configuring to indefinately. It's not a great solution because Caffeine also prevents your monitoring for turning off. So you waste a lot of energy / $$$.