I would discount polyphasic sleep as being natural on grounds of my current knowledge of anthropology. As far as I know there are no known human cultures that engage in polyphasic sleep (not counting biphasic sleep). That seems like pretty strong evidence that it isn't behavioral, it's physiological, which in turn suggests (but doesn't guarantee) an evolutionary basis for human sleep patterns. Of course, some amount of human sleep patterns is behavioral, e.g. the siesta.