biology

“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.

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 percent 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?

“Every time there is a claim that humans are special about something, once we start having more data, we realize they’re not that special,” Capellini says.

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.

“How is this possible, that we’re sleeping the least out of any primate?” he says. Sleep is known to be important for our memory, immune function and other aspects of health. A predictive model of primate sleep based on factors such as body mass, brain size and diet concluded that humans ought to sleep about 9.5 hours out of every 24, not seven.

“A lot of people in the global North and the West like to problematize 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.”