Humans sleep less than apes—lessons from a Tanzanian night

Humans sleep – A new evolutionary account argues that humans’ unusually short sleep and unusually REM-heavy nights weren’t a simple byproduct of modern life. Fieldwork with the Hadza hunter-gatherers suggests sleep improved when it became a communal, well-protected “social n

When the researcher’s weight settles into a chimpanzee nest 12 metres above the ground, the first feeling is relief. The nest seems built for the task: springy, cupped, stable—more furniture than pile of branches.

Then comes the jolt of reality. Even in Uganda’s Toro-Semliki Wildlife Reserve, the point isn’t just how animals rest. It’s how precarious sleep is. Sleep is biologically indispensable, critical for cognition, immune function, memory, attention and health. But it also leaves the sleeper extremely vulnerable—unable to defend itself or watch for danger—and it steals time that could be devoted to finding a mate or foraging.

That tension helps explain why humans are such an evolutionary outlier. and why a book-length argument about our “weird” sleep begins in a tree. The account—set out by author Michael Kirkham as he investigates one of life’s least appreciated evolutionary problems—centres on a paradox: humans sleep less than other primates. yet their sleep is built in a way that keeps a high share of REM. the stage tied to vivid dreaming. temporary muscle paralysis and wake-like brain activity.

Kirkham’s starting point is comparative. Across mammals. sleep duration varies drastically—from 2 hours a day for African elephants to 20 hours for the pocket mouse—and the pattern reflects messy tradeoffs shaped by predation risk. metabolic needs. foraging time and the safety of the sleeping site. Predators tend to sleep longer than prey animals.

But if you ask a biologist—armed only with body size. brain size. reproductive schedule. where a species sits on the primate family tree. and its typical slumber environment—to predict how much a primate like ours “should” sleep. the estimate is around 9.5 hours. After controlling for type of society (small off-grid versus large on-grid), humans cluster closer to an average of 7 hours. And the claim goes further: humans sleep less than any of the 30 primate species for which comparable data has been assembled.

That isn’t a small deviation that can be shrugged off as cultural tradition or modern overstimulation. In Kirkham’s framing, it is deep enough that any serious story about human evolution has to reckon with it.

The puzzle doesn’t end with how little humans sleep. It is also about how human sleep is constructed.

Sleep cycles through two main phases: REM sleep and non-rapid eye movement sleep (NREM). NREM is when the deepest sleep occurs. tied to physical restoration. strengthening of the immune system and consolidation of memories. especially declarative memories—those focused on recalling facts and events. REM. by contrast. is associated with vivid dreaming. temporary muscle paralysis and wake-like brain activity. where the brain is highly metabolically active while many skeletal muscles are switched off. REM is also linked to emotional processing and to integrating some memories and skills into brain pathways.

There is an added vulnerability during REM: thermoregulation—the process by which the body maintains its core temperature—is largely turned off because the brain temporarily prioritises dream-related neural activity and muscle paralysis over body-temperature control. In that state, body temperature becomes more dependent on the surrounding environment, leaving the sleeper “maximally compromised” in some ways.

So if evolution pushed humans toward less total sleep, why preserve such a costly stage?

Part of the answer. according to Kirkham’s account. comes from a 2018 study he carried out with Charles Nunn of Duke University in North Carolina. The study compared sleep characteristics of 30 primates, including chimps, orangutans, macaques and lemurs. Humans came out with the highest percentage of REM sleep among the primates sampled. That did not mean humans spend dramatically more time in REM than expected; human REM duration was only modestly above the predicted average.

The real shift, the study found, sits in reduced NREM. The model predicted 8.4 hours of NREM for humans, but the actual time typically spent in this phase was 5.4 hours. In effect, humans compress the night largely by cutting back on NREM while preserving a relatively large share of REM.

Kirkham describes this as an “odd situation.” If selection for reduced sleep time were driven by cost, the most expensive components might be expected to be trimmed first. Instead, humans became short sleepers while keeping the REM-rich architecture largely intact.

To make sense of it, he suggests the selection pressure might not have been about less sleep for its own sake. It may have been about a new kind of “sleep economy”: shorter, more efficient sleep that is reweighted toward cognitive functions that mattered in the lineage.

The setting where humans sleep—where bodies rest—plays an outsized role in that story.

Across primates. sleep sites determine concealment and safety: whether sleepers can be hidden. clinging safely. protected by a group. and how much danger results from becoming unconscious. Great apes solved some problems by building nests. In Kirkham’s account, those platforms likely improved sleep quality by increasing physical comfort and reducing predator danger.

He also points to specifics about trees favoured by chimpanzees. Some, like Cynometra or African ironwood, have mosquito-repellent properties. Barbara Fruth and Gottfried Hohmann of the Max Planck Institute for Animal Behaviour in Radolfzell. Germany. have even argued that nest-building created a cognitive leap for great apes. enabling better rest that could underpin the development of object-manipulation skills.

From that arboreal world. Kirkham’s narrative turns to the transition he says defined early humans: a move toward sleeping on the ground. From a chimpanzee nest’s perspective, the ground is a terrible trade. On savannah or woodland floors, sleepers are more accessible to predators and more exposed to hostile rivals.

Humans, he says, are the only primates that regularly snooze on terra firma across age and sex classes. Some male gorillas and some chimpanzees also choose the ground. but the risk of predator attack is described as low in those cases. The shift becomes part of the story by the time of Homo erectus around 1.8 million years ago.

That ground-based change is central to what Kirkham calls the human sleep paradox: a lineage apparently selected to compress sleep efficiently and under riskier conditions than its closest relatives. He argues this blocks easy explanations—humans aren’t simply “tired” because modern life is busy. and the short sleep average isn’t merely a cultural aberration tied to electricity. shift work. or glowing screens.

What matters instead is the pathway that got humans there in the first place.

In the account, the answer begins where the nest ends.

Chimpanzees, like other great apes, solved sleep problems by perfecting life in trees. Humans. Kirkham argues. solved the new vulnerability of ground sleep through cooperation—building shelter. fire. and shared vigilance into what he describes as a social sleep niche. In his view, sleep became something like collective infrastructure.

At some point in early Homo. the place where bodies slept became a collective technology: a defended patch of ground. a firelit perimeter. a shared shelter. and a zone of lowered risk created by many individuals at once. He frames this as a form of expanded influence beyond the body—what biologists call an extended phenotype. He extends the idea to sleep. introducing “sleep exophenotype”: the external sleep environment organisms create. modify or inherit in ways that alter the quality. safety. timing or expression of sleep.

In humans, he says, the exophenotype is communal—built out of bodies, technologies, habits and mutual dependence.

To describe the protective suite of innovations that humans wrap around sleep. he uses an acronym: SHELL. standing for shelter. hearth. environmental preparation. light and lookouts. The claim is that humans constructed a shell around sleep. and in doing so made terrestrial sleep workable while accommodating ever-increasing group sizes.

Kirkham connects this to his fieldwork among the Hadza hunter-gatherers in Tanzania, describing it as a pre-agriculture lens on sleep environments. His research assistant Ibrahim Mabulla and Kirkham lived and collaborated with the Hadza for a summer and used wearable devices to measure their sleep.

The measurements fed the argument directly. Group sleeping, he says, reduces the odds that everyone is equally vulnerable at once. People wake at different times across the night, distributing vigilance. Out of 20 nights measured, there were only 18 minutes when all the group members were asleep simultaneously. At least one person was awake at almost any given time throughout the night.

Kirkham describes the mechanisms behind that pattern: shelters and hearths that provide warmth. deterrence and a social centre of gravity; environmental preparation that includes clearing ground. organising bodies. reusing favoured camp locations and collectively shaping the wider sleep site so it is more predictable and defensible; and firelight that changes both visibility and evening sociality. Grass huts, he adds, buffer nighttime thermal variation.

This is where the “sleep economy” idea becomes practical rather than abstract. Once sleep is embedded in a shell. Kirkham says. social sleep could do something extraordinary: make sleep on the ground viable as communities grew. He also argues this would have let early humans gain cognitive benefits of shorter. more efficient. REM-rich sleep—such as improved creativity and innovation—without paying the full vulnerability costs.

He writes that once early humans could reliably manufacture warmer. quieter and safer nocturnal microhabitats. they were less bound to locations close to the equator and could eventually thrive in a wide range of ecologies. He frames this as a pre-adaptation for migration to latitudes away from equatorial Africa and says humans first had to learn how to carry a habitable night with them before colonising new continents.

So what does all of this mean for modern sleep problems?

Kirkham warns against a tempting conclusion: “going palaeo” by adopting hunter-gatherer habits like sleeping communally outside and abandoning mattresses, blackout curtains and sleep apps. He says the lesson from the Hadza is more subtle than a prescription to copy ancient routines.

In his framing, the Hadza aren’t “living fossils,” but East African hunter-gatherers living year-round in environments broadly similar to those faced by people in that region tens of thousands of years ago can still act as a comparative model for some ecological challenges early humans had to solve.

When Hadza adults report their nights. Kirkham says they typically describe a satisfying night’s rest. and only a small fraction respond that they don’t like their sleep. Yet, objectively, their nights don’t match an idealised Western fantasy of perfect rest. Like other foragers. they sleep relatively short hours by post-industrial standards. and their nights are often fragmented by awakenings. movement and noise.

So why does it seem to work so well?

One part of the answer. he suggests. is that the Hadza may not be sleeping more. but they may be sleeping more in sync with their environment. Their sleep patterns are attuned to daily cycles of lightness and darkness. helped by brighter days. darker nights and more regular daily rhythms than those typically seen in industrialised societies. That shows up in a “higher circadian function index. ” a measure of how well a person’s circadian rhythms align with the prevailing light conditions.

That stronger alignment, he argues, may help their sleep feel better even if it is shorter and broken up. It may also help explain the paradox of Hadza sleep: people can tolerate a shorter and more interrupted night if sleep falls at the right time relative to their body’s rhythms.

Kirkham’s conclusion returns to the emotional edge that started in the chimpanzee nest: the goal isn’t to recreate an ancestral sleep schedule as a nostalgia project. Instead. he says. it is to recover circadian synchrony that modern routines have made easy to lose—without obsessing over “lost sleep. ” and without treating length as the only measure of recovery.

In the end, the nest and the ground point to the same idea: sleep isn’t only a biological state. It is also a built arrangement—by groups, by firelight, by timing, by the world you share at night.

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