And so Yetish suggests that ancient humans may have traded some hours of sleep for sharing information and culture around a dwindling fire. “You’ve suddenly made these darkness hours quite productive,” he says. Our ancestors may have compressed their sleep into a shorter period because they had more important things to do in the evenings than rest.
How much we sleep is a different question, of course, from how much we wish we slept. Samson and others asked Hadza study participants how they felt about their own sleep. Out of 37 people, 35 said they slept “just enough,” the team reported in 2017. The average amount they slept in that study was about 6.25 hours a night. But they awoke frequently, needing more than 9 hours in bed to get those 6.25 hours of shut-eye.
By contrast, a 2016 study of almost 500 people in Chicago found they spent nearly all of their time in bed actually asleep, and got at least as much total sleep as the Hadza. Yet almost 87% of respondents in a 2020 survey of US adults said that on at least one day per week, they didn’t feel rested.
Why not? Samson and Yetish say our sleep problems may have to do with stress or out-of-whack circadian rhythms. Or maybe we’re missing the crowd we evolved to sleep with, Samson says. When we struggle to get sleep, we could be experiencing a mismatch between how we evolved and how we live now. “Basically we’re isolated, and this might be influencing our sleep,” he says.
A better understanding of how human sleep evolved could help people rest better, Samson says, or help them feel better about the rest they already get.
“A lot of people in the global North and the West like to problematise their sleep,” he says. But maybe insomnia, for example, is really hypervigilance — an evolutionary superpower. “Likely that was really adaptive when our ancestors were sleeping in the savannah.”
Yetish says that studying sleep in small-scale societies has “completely” changed his own perspective.
“There’s a lot of conscious effort and attention put on sleep in the West that is not the same in these environments,” he says. “People are not trying to sleep a certain amount. They just sleep.”
* This article originally appeared in Knowable Magazine, and is republished under a Creative Commons licence.
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