Why do humans sleep less than primates? On dry nights, the San hunter-gatherers of Namibia often slept under the open sky. They have no electric light or access to modern technology to keep them awake. However, they do not sleep more than people in the West.
Research has shown that people in non-industrial societies sleep an average of less than seven hours a night, according to evolutionary anthropologist David Samson of the University of Toronto Mississauga. This is a surprising number when you consider our closest relatives, animals.
Humans sleep less than any monkey or lemur that scientists have studied. Chimpanzees sleep around 9.5 hours 24. Cotton tamarins sleep around 13. Night monkeys are technically nocturnal, although they sleep 17 hours a day, points out Smithsonian Magazine.
Samson calls this discrepancy the paradox of human sleep. “How is this possible, that we sleep the least of the primates?” 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 should sleep about 9.5 hours out of 24, not seven. “Something strange is happening,” says Samson.
Why do people sleep less?
Research by Samson and others on non-industrial primate and human populations has revealed the various ways in which human sleep is unusual. We spend fewer hours sleeping than our closest relatives, and much of our nighttime sleep is REM. The reasons for our strange sleeping habits are still debated, but they can probably be found in the story of how we became human.
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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 the branches to create a bowl shape, which they can line with leafy twigs. Mammoths like gorillas also sometimes build beds on the ground.
Our ancestors transitioned from the trees to living on the ground, and at some point they started sleeping there too. This meant giving up all the advantages of tree-sleeping, including relative safety from predators such as lions. The fossils of our ancestors do not reveal how well they rested. So to find out how ancient people slept, anthropologists study the closest intermediate link they have at hand: contemporary non-industrial societies.
From four-poster bed to “snail shell”
“It’s an amazing honor and opportunity to work with these communities,” says Samson, who has worked with the Hadza hunter-gatherers in Tanzania, as well as various groups in Madagascar, Guatemala and elsewhere. Study participants generally wear a device called an Actiwatch, which is similar to a Fitbit with an added light sensor, to record their sleep patterns.
Gandhi Yetish, an ecologist and anthropologist at the University of California, Los Angeles, has also spent time with the Hadza, as well as the Tsimane of Bolivia and the San of Namibia. In a 2015 paper, he assessed sleep in all three groups and found that they averaged just 5.7 and 7.1 hours.
Therefore, humans seem to have evolved to need less sleep than our primate relatives. Samson showed in a 2018 review that we achieved this by cutting non-REM time. REM is the sleep phase most associated with lucid dreaming. This means that, assuming other primates dream similarly, we may spend a greater proportion of our night dreaming than they do. We’re also flexible about when we have those shut-eye hours.
We evolved to need less sleep
“We should think of early human camps and groups as a snail shell,” he says. Groups of people may have shared simple shelters. Fire could have kept people warm and bugs away. Some members of the group may be sleeping while others are awake.
It makes sense that the threat of predators would have led humans to sleep less than tree-dwelling primates, says Isabella Capellini, an ecologist at Queen’s University Belfast in Northern Ireland. In a 2008 study, she and her colleagues found that mammals with a higher degree of vulnerability slept less on average. But Capellini isn’t sure human sleep is so different from that of other primates. She points out that the existing data on sleep in primates comes from captive animals. “We still don’t know much about how animals sleep in the wild,” she says.
In a zoo or laboratory, animals may sleep less than is natural due to stress. Or they might sleep more, Capellini says, “just because the animals are so bored.” And standard laboratory conditions—12 hours of light, 12 hours of darkness—might not match what an animal experiences in nature year-round.
Fireside chats
Yetish, who studies sleep in small-scale societies, collaborated with Samson on the research. “I think social sleep, as he describes it, is a solution to the problem of staying safe at night,” says Yetish. However, he adds: “I don’t think it’s the only solution.”
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He notes that the Tsimane people sometimes build walls on their houses, for example, which would provide some security without human vigilance. Sleeping in groups, predator threats or not, is also a natural extension of how people in small societies live during the day, Yetish says. “In my opinion, people are almost never alone in these kinds of communities.”
And so, Yetish suggests that ancient people may have traded a few hours of sleep to share information around the fire. “Suddenly you’ve made these dark 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 evening than rest.
An unsatisfactory sleep
How much we sleep is a different matter compared to how much we would like to sleep. Samson and others asked Hazda study participants how they felt about their sleep. Of 37 people, 35 said they got “just enough” sleep, the team reported in 2017. The average time they slept was about 6.25 hours a night. But they woke up frequently, needing more than 9 hours in bed to get those 6.25 hours of shut-eye.
In contrast, a 2016 study of nearly 500 people in Chicago found that they spent almost all of their time in bed sleeping and slept at least as much as the Hadza. However, nearly 87 percent of respondents to a 2020 survey of US adults said they did not feel rested at least one day a week.
Why not? Samson and Yetish say our sleep problems could be related to stress or irregular circadian rhythms. Or maybe we miss the community we used to sleep in, says Samson. When we struggle to sleep, we might experience a mismatch between how we evolved and how we live now. “We’re basically isolated, and that could affect our sleep,” he says.