I'm sure it depends on your optimality criterion. I have looked into the research behind this two-sleep phenomena, and it seems like a highly plausible theory for human before before the last couple hundred years. One might expect this to be close to some kind of evolutionary optimum characteristic of the last 3000 or so years that most of humanity has lived in cities. This environment, I think, should be quite different from our society where we have things like "hackers" and are responding to interesting comments from the internet at one in the morning (maybe I should go to bed).