If we take the nocturnals at their word—that they simply like living this way—they complicate one of our core assumptions about human psychology: that all people have the same fundamental needs.
This basic problem, assuming some base commonality that turns out to have a much larger variability within a "normal" population, seems to plague a lot of the results coming out of the social sciences. It's one of the primary reasons I don't put a lot of weight into the research in these fields. I often find my lived experience very different to what they find. Maybe it is a necessary step on the road to a fuller understanding (we've got to start somewhere), but I think we end up over-simplifying the complexities of what it can mean to be human.