Te upshot of sleep’s preindustrial and postindustrial history is a simple, short, and consistent message: Sleep is adaptable, but it improves with routine. Different tricks work for different tribes, but in the end, we are a diverse species united by a common circadian rhythm that craves consistency.

There is no evidence that sleep was universally segmented, and there is also little evidence that segmented sleep is better. A 2021 meta-analysis of studies on biphasic sleep schedules found that segmented-sleeping subjects actually reported “lower sleep quality … and spent more time in lighter stages of sleep.” One reasonable takeaway is that... See more