Interesting spam trend

So I was looking at filter stats, and noticed an interesting trend on the spam time of day average graph. There's a base level of junk from knows sources, the level of which doesn't change much. But then there's an additional amount which has an interesting slope over the day. It's like the spammers wake up with a new set of accounts, machines and connections, and then it gets cut off as ISPs find it and take action. It seemed interesting.