This is total speculation on my part, but it seems like there would be a strong evolutionary pressure, at a group level, to select for a mixture of night owls a

This is total speculation on my part, but it seems like there would be a strong evolutionary pressure, at a group level, to select for a mixture of night owls and early risers. The "night owls" stay up to watch for predators to keep the group safe. Those groups without some late sleeper DNA ultimately get selected out.