We traded sleep for the cognitive and social advantages that made us human.
For decades, the story of human sleep has been one of loss — stolen hours blamed on screens, stress, and the relentless pace of modern life. But evolutionary anthropologist David Samson offers a quieter, older explanation: humans have slept less than other primates for nearly two million years, not because civilization broke us, but because natural selection shaped us this way. Across hunter-gatherer societies and industrialized nations alike, the data points not to a crisis of duration, but to a deeper question of alignment — whether our rest moves in harmony with the rhythms the world has always offered us.
- Humans sleep roughly 4.5 hours less per night than their body and brain size would predict, making them the least-sleeping primates on Earth — yet among the longest-lived and most cognitively capable.
- The modern narrative blaming smartphones and work stress for sleep deprivation collapses when confronted with hunter-gatherer societies like the Hadza and Himba, who sleep even less yet maintain stronger health and circadian rhythms.
- The evolutionary trade was deliberate over 1.8 million years: reduced non-REM sleep freed hours for social bonding, tool-making, and the cognitive leaps that defined humanity — sleep was not lost, it was exchanged.
- A global study across thirty countries confirms that both too little and too much sleep carry health risks, suggesting the real variable is not quantity but how closely a person's rest aligns with natural light and temperature cycles.
- The path forward, Samson argues, is not sleep optimization through technology or longer hours in bed, but a 'Sleep Enlightenment' — learning to synchronize rest with the biological and environmental rhythms that shaped us.
We have been told for years that we are in a sleep crisis — that modern life is stealing our rest. In Spain alone, sleeping pill consumption has tripled in under two decades. But David Samson, an evolutionary anthropologist at the University of Toronto, turns this narrative on its head: humans sleep less not because stress has broken us, but because evolution designed us that way.
Samson spent years observing primates in the wild and living alongside hunter-gatherer communities — the Hadza in Tanzania and the BaYaka in the Republic of Congo. What he found was striking. Analyzing sleep across more than thirty primate species, his models predicted humans should sleep around eleven and a half hours per night based on our body and brain size. Instead, we average seven — and the Hadza average just 6.4, with fragmented, restless nights. The Himba of Namibia sleep roughly four and a half hours, yet show excellent cardiovascular and mental health. If smartphones and anxiety were the culprit, these populations should be sleeping soundly. They are not.
The shift began approximately 1.8 million years ago, when early humans built shelters and discovered fire. Cooking reduced the hours spent chewing from five or six down to one, freeing time that was reinvested in socializing, learning, and the cognitive leaps that made us human. Natural selection gradually trimmed non-REM sleep, trading rest for connection and capacity.
What Samson emphasizes is that duration is the wrong measure. The critical variable is alignment — whether sleep occurs in sync with natural light, temperature, and circadian rhythms. A consistent global study found that sleeping too much is as harmful as sleeping too little; the optimal amount varies by culture and environment. Even napping follows this logic: twenty to thirty minutes can restore, but longer naps erode the sleep pressure needed for a good night's rest.
The future Samson envisions is not one of sleep hacks or longer hours in bed, but of understanding — a Sleep Enlightenment in which we stop fighting our evolutionary nature and instead learn to rest the way our biology, shaped across nearly two million years, has always intended.
We have been told for years that we are sleeping less, and that this is a crisis. The Spanish Society of Neurology estimates that more than half of Spanish adults sleep fewer hours than recommended, and nearly half report their rest is not restorative. Sleeping pill consumption has tripled in less than two decades. The narrative is familiar: modern life—long work hours, constant digital connection, anxiety, relentless pace—is stealing our sleep. But what if that story is backwards?
David Samson, an evolutionary anthropologist at the University of Toronto, proposes something counterintuitive: humans sleep less not because our lives have become unbearably stressful, but because we have evolved to need less sleep. The evidence, he argues, is written into our biology and into the lives of the people he has studied. Samson spent years observing chimpanzees, orangutans, and lemurs in the wild. He also lived among two hunter-gatherer populations—the Hadza in Tanzania and the BaYaka in the Republic of Congo—sleeping alongside them, watching their rhythms. What he found was striking: humans are the least-sleeping primates on Earth, yet we live longer, healthier lives with greater cognitive capacity than any other primate species. This is what he calls the paradox of human sleep.
The numbers tell the story. When Samson and his colleagues analyzed sleep patterns across more than 30 primate species, controlling for evolutionary history, body size, brain size, diet, and social structure, they discovered something unexpected. Based on our physical and neurological characteristics, the mathematical model predicted that humans should sleep approximately eleven and a half hours per day. Instead, across cultures worldwide—both small-scale societies and large industrialized nations—humans average around seven hours. We have undergone, Samson argues, a radical evolutionary experiment that made us the least-sleeping primates of our entire order.
But here is where the modern stress narrative collapses. The hunter-gatherer societies Samson studied sleep even less than we do in the industrialized world. The Hadza and Baka average 6.4 hours per night, and their sleep is far more fragmented than ours. If our sleep deprivation were caused by smartphones, artificial light, and demanding jobs, these populations should be sleeping soundly. They are not. Instead, they are sleeping less and sleeping worse—yet they are healthier. Their circadian rhythms, their internal biological clocks, are stronger and more tightly synchronized with their environment. The Himba people of Namibia sleep only about four and a half hours per night on average, yet their cardiovascular health, mental health, and resistance to modern diseases are remarkably good.
The shift happened roughly 1.8 million years ago, when humans began building shelters. Over hundreds of thousands of years, natural selection gradually reduced our sleep duration, specifically cutting back on non-REM sleep. At the same time, the discovery of fire transformed how we ate. Chimpanzees spend five or six hours a day chewing; gorillas spend eleven. Humans, with cooked food, reduced that to about one hour. We gained time. We lost sleep, but we gained something more valuable: hours for socializing, for strengthening bonds, for hunting, for learning to carve stone tools and use weapons. We traded sleep for the cognitive and social advantages that made us human.
What matters, Samson emphasizes, is not simply how many hours you sleep but when you sleep and whether your rest aligns with natural light and temperature cycles. A study published in PNAS analyzing sleep and health across thirty countries found a consistent pattern: sleeping too little is harmful, but sleeping too much—ten or eleven hours—is also harmful. The optimal amount varies by culture. Some cultures thrive on seven or eight hours; others maintain perfect health on six. The difference, Samson hypothesizes, lies in how closely their light exposure, temperature fluctuations, and daily rhythms match their environment. When your body's independent clocks are synchronized with the world around you, less sleep suffices.
The question of whether we can improve our sleep, then, is not primarily about duration. It is about alignment. A nap of twenty to thirty minutes can be beneficial if you are sleep-deprived, but longer naps can be counterproductive, reducing the homeostatic pressure that builds sleep drive throughout the day. For those struggling with insomnia, the advice is paradoxical: skip the nap, wake earlier, go to bed later, and let that pressure accumulate. The future of sleep, Samson suggests, lies not in fighting our evolutionary nature but in understanding it—in what he calls a coming age of Sleep Enlightenment, when we stop trying to hack our rest and instead align it with the rhythms that shaped us.
Notable Quotes
We have undergone a radical evolutionary experiment that made us the least-sleeping primates of our entire order.— David Samson, evolutionary anthropologist
What matters is not simply how many hours you sleep but when you sleep and whether your rest aligns with natural light and temperature cycles.— David Samson
The Hearth Conversation Another angle on the story
If humans sleep less than other primates, why haven't we simply collapsed from exhaustion? What changed in our biology?
We didn't collapse because we gained something in return. When we started using fire and building shelters about 1.8 million years ago, natural selection began trimming our sleep budget—specifically the non-REM sleep. But we didn't lose cognitive function. We gained time. Hours that used to go to sleep went to socializing, tool-making, hunting, learning. The trade was worth it.
But the data from Spain shows half of adults aren't sleeping well. Isn't that a real problem?
It is, but not for the reason most people think. The problem isn't that we're sleeping seven hours—that's actually optimal for our species. The problem is that we're sleeping seven hours at the wrong times, under the wrong light conditions, disconnected from our environment. A hunter-gatherer sleeping 6.4 hours is healthier than an industrialized person sleeping eight because their sleep is synchronized with natural rhythms.
So the solution isn't more sleep. It's better sleep.
Exactly. And better sleep means understanding when and how your body wants to rest. Some people have genetic variants that let them function well on five hours. Most of us need closer to seven. But all of us need our internal clocks aligned with the world—real darkness at night, real light in the morning, temperature shifts. That's what we've lost, not sleep itself.
You mentioned the Himba sleep only four and a half hours. How is that possible without cognitive decline?
Because their circadian rhythms are incredibly strong. Their bodies are not separated from their environment the way ours are. They're not fighting artificial light at night or sitting under fluorescent bulbs all day. Their biological clocks are locked into the natural world. That synchronization does something sleep duration alone cannot.
What about the siesta? Is that a relic of our evolutionary past or just a cultural habit?
It's complicated. A short nap—twenty to thirty minutes—can help if you're genuinely sleep-deprived. But longer naps can actually undermine your sleep drive, making it harder to sleep at night. And if you have insomnia, napping can be counterproductive. The siesta works best when it's a supplement to good nighttime sleep, not a replacement for it.
You say we're heading toward a 'Sleep Enlightenment.' What does that look like?
It means stopping the obsession with optimizing sleep duration and starting to understand sleep as something that needs to be in rhythm with your life and your environment. It means recognizing that seven hours is not a failure—it's what we evolved for. The question isn't how to sleep more. It's how to sleep in sync.