One could see where there's a biological advantage in this pattern. For a group of pre-modern humans, most tasks will happen during daylight, but some (keeping

One could see where there's a biological advantage in this pattern. For a group of pre-modern humans, most tasks will happen during daylight, but some (keeping watch, childcare, tending a fire) work best with someone awake around the clock. Notably, those tasks can be done by people who aren't either physically able enough (older) or mentally experienced enough (younger) to do the core activities (hunting and gathering). With older people waking up very early, and adolescents staying up very late, you can cover the entire daily cycle with alert humans capable of doing basic tasks (and alerting the rest of the group should something come up).