This article was originally published by Knowable Magazine.
On dry nights, the San hunter-gatherers of Namibia often sleep under the stars. They have no electric lights or new Netflix releases keeping them awake. Yet when they rise in the morning, they haven’t gotten any more hours of sleep than a typical Western city dweller who stayed up doomscrolling on their smartphone.
Research has shown that people in nonindustrial societies—the closest thing to the kind of setting our species evolved in—average less than seven hours a night, says David Samson, an evolutionary anthropologist at the University of Toronto, Mississauga. That’s a surprising number when you consider our closest animal relatives. Humans sleep less than any ape, monkey, or lemur that scientists have studied. Chimps sleep about nine and a half hours out of every 24. Cotton-top tamarins sleep about 13. Three-striped night monkeys are technically nocturnal, though, really, they’re hardly ever awake—they sleep for 17 hours a day.
Samson calls this discrepancy the human sleep paradox. “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 nine and a half hours out of every 24, not seven. “Something weird is going on,” Samson says.
Research by Samson and others in primates and nonindustrial human populations has revealed the various ways that human sleep is unusual. We spend fewer hours asleep than our nearest relatives, and more of our night in the phase of sleep known as rapid eye movement, or REM. The reasons for our strange sleep habits are still up for debate but can likely be found in the story of how we became human.
Millions of years ago, our ancestors lived, and probably slept, in trees. Today’s chimpanzees and other great apes still sleep in temporary tree beds or platforms. They bend or break branches to create a bowl shape, which they may line with leafy twigs. (Apes such as gorillas sometimes also build beds on the ground.)