On dry nights, hunter-gatherers from the San tribe in Namibia often sleep under the stars.
They don't have electric lights or new series on Netflix, the online platform, to keep them awake.
Then again, when they get up in the morning, they haven't slept any more than the average resident of a big city in North America or Europe who stayed up reading defeatist news on a smartphone.
Research has shown that people in non-industrial societies, the closest to the environment in which our species evolved, sleep an average of less than seven hours a night, says evolutionary anthropologist David Samson of the University of Toronto Mississauga.
That's surprising when you consider our closest animal relatives.
Humans sleep less than any ape, ape or lemur that scientists have studied.
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Chimpanzees sleep about 9.5 hours every 24 hours.
Cotton tamarins sleep for about 13 hours.
Three-striped night monkeys are technically nocturnal animals, although they are actually rarely awake at all — sleeping 17 hours a day.
Samson calls this discrepancy the paradox of the human dream.
"How is this possible, that we sleep less than any primate?" he says.
It is known that sleep is an important part of our memory, immune function and other aspects of our 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 every 24 hours, rather than seven.
"Something strange is going on there," says Samson.
Research by Samson and others on primates and non-industrial human populations has shown in various ways that human sleep is unusual.
We spend fewer hours sleeping than our closest relatives and most of the night in a stage of sleep known as rapid eye movement, or REM.
The reasons for our unusual sleeping habits are still a matter of debate, but they may be found in the story of how we became human.
How did our ancestors sleep?
Millions of years ago, our ancestors lived, and probably slept, in trees.
Today's chimpanzees and other great apes still sleep on temporary beds in trees or on platforms.
They bend or break the branches into a bowl shape, which they can place flush with the leafy branches.
(Great apes such as gorillas also sometimes sleep on the ground.)
Our ancestors came down from the trees to live on the ground and at some point started sleeping on it.
This meant giving up all the benefits of arboreal sleep, as well as relative safety from predators such as lions.
The fossils of our ancestors do not reveal how rested they were.
And so, to find out how early humans slept, anthropologists study the best intermediary they have: modern non-industrial societies.
"It's a tremendous honor and opportunity to work with these communities," says Samson, who has worked with the Haj hunter-gatherers of Tanzania, as well as various other groups in Madagascar, Guatemala and elsewhere.
Study participants generally wear a device called an Aktiwatch, similar to a Fitbit with an added light sensor, that records their sleep patterns.
Gandhi Yetish, a human evolutionary ecologist and anthropologist at the University of California, Los Angeles, has also spent time with the Hajj tribe, as well as the Tsimane in Bolivia and the San in Namibia.
In a 2015 study, he evaluated the sleep of all three groups and found that they slept an average of between 5.7 and 7.1 hours.
Humans, then, seem to have evolved to need less sleep than our primate relatives.
Samson showed in a 2018 analysis that we did this by shortening our non-REM phase.
REM is the stage of sleep most associated with vivid dreaming.
This means that, assuming other primates dream similarly, we may spend more time during the night dreaming than they do.
We are also more flexible about when we get those hours off.
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To tie together the story of how the human dream developed, Samson laid out what he calls the 2021 Social Dream Hypothesis in the Annual Review of Anthropology.
He believes that the evolution of human sleep is a story about safety - specifically, safety in large numbers.
Short, flexibly timed REM-laden sleep probably evolved because of the threat of predators when humans started sleeping on the ground, Samson says.
And he thinks another key to sleeping safely on the ground is group napping.
"We have to think of early human camps and groups as a snail's house," he says.
Groups of people may have shared simple shelters.
The fire probably kept the people warm and kept the bugs at bay.
Some members of the group were able to sleep while others kept watch.
"Within the safety of that social shell, you could go back and take a nap at any time," imagines Samson.
(He and Jetish disagree about the frequency of napping in today's non-industrial groups. Samson cites frequent napping in the Hajj tribe and the Madagascar population. Jetish says that, based on his experience in the field, napping is not frequent.)
Samson also thinks that these sleeping shells allowed our ancient ancestors to travel from Africa to colder climates.
In this way, he sees sleep as a key subplot in the story of human evolution.
It makes sense that the threat of predators would cause humans to sleep less than tree-dwelling primates, says Izabella Capellini, an evolutionary ecologist at Queen's University in Belfast, Northern Ireland.
In a 2008 study, she and her colleague found that mammals that are more threatened by predators sleep less on average.
But Capellini is not sure that human sleep is as different from the sleep of other primates as it seems at first glance.
She points out that existing data on sleep in primates comes from captive animals.
"We still don't know much about exactly 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, she says, adding "just because the animals are very bored."
And the standard conditions in laboratories, 12 hours of light, the same amount of darkness, may not match the animal's experience in nature during the year.
Neuroscientist Neil Rattenborg, who studies bird sleep at the Max Planck Institute for Ornithology in Germany, agrees with Samson's version that the evolution of human sleep is interesting.
"I think a lot depends on whether we measure sleep accurately in other primates," he says.
And there is reason to suspect that we are not.
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In a 2008 study, Rattenborg and colleagues attached electroencephalography (EEG) devices to three wild sloths and found that these animals slept about 9.5 hours a day.
An earlier analysis of captive sloths, on the other hand, recorded almost 16 hours of sleep per day.
Data from more wild animals would help sleep researchers.
"But it's technically a big challenge to achieve that.
"Although the sloths were receptive to this procedure, I have a feeling that the primates would spend a lot more time trying to get their gear off," says Rattenborg.
If scientists had a clearer picture of primate sleep in the wild, it might turn out that human sleep isn't as uniquely short as thought.
"Every time it's claimed that humans are special about something, once we start getting more data, we realize that we're not that special after all," Capellini says.
Jetiš, who studies sleep in small societies, collaborated with Samson on the research.
"I really think that social sleep, as he describes it, is the solution to the problem of maintaining security at night," says Jetis.
However, he does not think that this is the only solution.
He points out that the Tsimane sometimes have walls on their houses, for example, which would provide some security without a human guard.
And to Jetisha, the people in the groups he studies mentioned in the morning exactly which animals they heard during the night.
The sounds wake most people at night, offering another possible layer of protection.
Sleeping in groups, whether there is a threat from predators or not, is also a natural extension of the way people in small societies live during the day, Jetis says.
"In my opinion, people are almost never alone in these types of communities."
Yetiš describes a typical evening in the Tsimane tribe: after a day spent working on various tasks, the group gathers around a fire while food is prepared.
They share a meal, then hang out around the fire in the dark.
Children and mothers then gradually go to sleep, while the others stay awake, talking and telling stories.
And that's why Jetiš suggests that early humans may have sacrificed a few hours of sleep for the sake of exchanging information and culture around the campfire.
"Suddenly you'd turn these hours of darkness into something quite productive," he says.
Our ancestors may have compressed sleep into a shorter period because they had more important things to do in the evening than rest.
How much we sleep is a different question, of course, than how much we would like to sleep.
Samson and others asked members of the Hajj tribe who participated in the study about their own dreams.
Of the 37 people, 35 said they got "just enough" sleep, the team reported in 2017.
The average amount of sleep they got in that study was 6.25 hours a night.
But they would wake up often, so it took them more than 9 hours in bed to finally get those 6.25 hours of rest.
In contrast, a 2016 study of nearly 500 people in Chicago found that they spent almost all of their time in bed actually sleeping, and that they got at least as much total sleep as members of the Hajj.
And yet, almost 87 percent of respondents in a 2020 study of American adults said they don't feel rested at least one day a week.
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Samson and Jetiš say that our sleep problems may have something to do with stress and a misaligned circadian rhythm.
Or maybe we're missing the mass of people we evolved to sleep with, says Samson.
When we have trouble sleeping, we may be experiencing a mismatch between how we evolved and how we live today.
"We're practically isolated, and that can affect our sleep," he says.
A better understanding of how human sleep evolved could help people get better rest, Samson says, or help them make better use of the rest they already have.
"A lot of people in the global north and west like to problematize their dream," he says.
But maybe insomnia, for example, is just hypervigilance, an evolutionary superpower.
"That was probably very useful when our ancestors slept on the savannah."
Jetis says that studying sleep in small societies "completely" changed his own perspective on the issue.
"There's a lot of conscious effort and attention paid to sleep in the West compared to those environments," he says.
"People don't try to sleep a certain amount of time. They're just sleeping."
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