Mammals have internal circadian rhythms. Humans will drift up about 24.5 hours on average without sun, with significant variance.

We are amazing at syncing our clocks do another system, though I don't know how we'd handle 48 hours.

Separately, you assume we've always were 16/8. South America, mostly Mexico, used to an extended rest in the day time. Before the industrial revolution, many cultures would sleep from dusk till midnight, get up for a few hours and do community activities, and then sleep until early morning.