Our general biological rhythms are longer than 24 hours. If you remove these light and dark hues, which some chronobiologists have done by literally putting people like underground, eventually their whole schedule starts to shift. In the course of a person's lifetime, there is almost the same pattern, a peak, a trough and then a bit of recovery over the span of a natural human life. You can find this pattern in something like 70 different countries. So it isn't simply a phenomenon of the united states or north america or the west. There's probably something quotan natural about it, but that nature, as with all things, interacts with circumstance and experience.