It's usually around the afternoon, when the suits are at work, and they assume we are all at work as well, so that's where they target most of their high paying keywords I think.
My earnings seem to follow a regular patern and from 1600 on i don't earn very much with adsense. I think this has to do with daily budgets though. Most people's budgets run out and then you seem to get very low CPC at the end of the day from the people who have a low min CPC. Just a guess.
i would have to say i have peak around lunch time 11 - 1 and then again right after people get off work 5 -7.