To learn about how ancient humans slept, anthropologists study contemporary non-industrial societies, such as the Hadza hunter-gatherers of Tanzania and various groups in Madagascar, Guatemala, and elsewhere. Sleep across all three groups averages around 5.7 - 7.1 hours, which is much less than primates.

Evolutionary anthropologist David Samson thinks the evolution of human sleep is a story about safety in numbers when humans began sleeping on the ground and napping often.

The threat of predators may have led humans to sleep less than tree-living primates. Yet, scientists point out that existing data about sleep in primates come from captive animals, who may sleep more because the animals are bored.

Having data from more wild animals would help sleep research...