I thought I had read it on Hacker News like a year ago or something. But I just spent 5 minutes Googling, and I can't find it now! argh. Keywords were variations of "early late risers dimorphism predators". I will look a little more later.
I don't see anything there related to the hypothesis that some people are early risers and others late risers so that someone is awake at all times. It seems to be the source of this segmented sleeping phenomena, which is different although very interesting too.