Sleep is something humans have always done well — until someone decided there was a right way to do it. The research behind this piece covers the full story. This is the plain-language version.
Most people think of sleep as restoration — the body repairing itself overnight, like a phone on charge. That account is partly right. Things do happen during sleep that are useful: the brain clears waste, memories get processed, hormones pulse. But researchers who have looked at the deeper question — why does sleep exist at all — have arrived at a more fundamental answer.
Sleep is, at its core, about saving energy. When there is nothing useful to be gained from being active — no food to find, no threat to respond to — the most sensible thing a body can do is cut its running costs right down and wait. During sleep, heart rate slows, temperature drops, breathing quietens. The body ticks over on a reduced budget until being awake is worth it again.
This matters because it changes what counts as normal sleep. If sleep is a fixed biological requirement — a specific number of hours that must be met in a specific way — then any deviation from it becomes a problem. But if sleep is a flexible energy-saving strategy, the body adjusts how much it takes and in what form depending on what the situation actually needs. And the natural world makes it very clear that this is exactly how sleep works.
Wild African elephants sleep for about two hours a day. The same species in captivity, with no predators and no distances to cover, sleeps considerably more. Nothing changed in the biology. The situation changed. Great frigatebirds, during non-stop ocean crossings lasting up to ten days, sleep for around forty minutes a day — sometimes with just one half of the brain at a time, the other half keeping them in the air. Bottlenose dolphins sleep with one brain hemisphere at a time so the other can keep them surfacing and breathing.
None of these animals are following a rule about how much sleep they need. They are all doing the same thing: taking the sleep their circumstances allow, in the form that fits.
The spectrum extends further still. Hummingbirds enter a state of near-suspended animation overnight — body temperature dropping dramatically, heart rate plummeting — because the energy cost of staying warm through the night at their size is simply too high. Hibernating bears give birth and nurse cubs while barely eating for months, their bodies selectively maintaining what matters most while withdrawing from everything else. The African lungfish burrows into a drying riverbed, seals itself in a mucus cocoon, and survives for up to four years without food or water, its heart beating twice a minute.
These are not freaks of nature. They are expressions of the same underlying principle — that the body withdraws from the cost of being active when being active offers nothing in return, and does so in whatever form its circumstances demand. Sleep, daily torpor, hibernation, and suspended animation are not separate biological states. They are points on a single spectrum of the same adaptive strategy.
For most of human history, people did not sleep in one consolidated block overnight. They slept in two separate stretches, with a quiet wakeful period in between — sometimes an hour, sometimes two. The historian Roger Ekirch spent years going through old diaries, court records and medical texts from across Europe and found this pattern — the "first sleep" and the "second sleep" — running consistently through several centuries of everyday life. Nobody wrote about it as unusual, because it wasn't. It was simply how nights worked.
A scientist called Thomas Wehr later removed artificial light from participants' lives for several weeks and found that most of them quietly drifted back to the same pattern. The body, given the chance, returned to it on its own.
Research on hunter-gatherer communities living today without industrial schedules or electric light tells the same story. Three communities studied — the Hadza in Tanzania, the San in Namibia, and the Tsimane in Bolivia — averaged between 5.7 and 7.1 hours of sleep a night. None of them showed the health problems that modern sleep guidance predicts for anyone sleeping less than eight hours. They napped when they felt like it, slept differently across seasons, and had no fixed bedtime. Their sleep looked nothing like the contemporary Western standard — and they were fine.
Farming communities across Britain and northern Europe worked through harvest moon nights for centuries — planned, communal, physically demanding all-nighters at the busiest time of their agricultural year. The biology absorbed it. They came back the following year and did it again.
The consolidated eight-hour overnight block was not discovered by scientists. It was produced by factories.
When industrial production started running around the clock, it needed workers at fixed, predictable times. Shift work required regular sleep schedules that could be managed and timed. Gas lighting, and then electric light, progressively extended the available working day and compressed the old natural rhythms of the night — the two-sleep pattern, the quiet wakeful interval, the seasonal variation — into a single manageable block that fitted the factory rota. The eight-hour target emerged from the campaign for regulated working hours in the nineteenth century. It was as much an industrial convenience as a biological recommendation.
Over time, the norm became so thoroughly embedded that its industrial origins became invisible. Researchers studied populations that had already internalised it. Deviation from it looked pathological, because the norm had become the baseline. Insomnia as a medical category expanded to include what had previously been an unremarkable feature of the human night.
The modern sleep anxiety industry found its most powerful voice in a 2017 book called Why We Sleep, by a scientist called Matthew Walker. Its message was hard to miss: sleeping less than eight hours causes cancer, dementia, heart disease, obesity, and mental illness. The NHS and health charities amplified it. Sleep became a civilisational crisis.
The research Walker drew on is real. Sleep does matter. But his book was subsequently examined in detail by other researchers, who found a significant number of places where the science had been presented more alarmingly than the underlying studies warranted — associations described as causes, risks stated at the high end of the plausible range without acknowledging the uncertainty around them. The critique was not fringe commentary. It was a documented academic challenge from within the field.
The core problem is the application. The evidence that severe, prolonged sleep deprivation causes harm is credible. The evidence that sleeping six hours instead of eight, or waking briefly in the night, or a baby not consolidating overnight sleep by a specified age, causes the same harm — that evidence is considerably weaker than the messaging implies.
Professor Helen Ball, who leads the Durham Infancy and Sleep Centre and has spent over two decades researching how babies actually sleep, has consistently found that the range of normal in infant sleep is far wider than clinical guidance suggests. The charts and milestone tables that tell parents their baby should be sleeping through by a certain age reflect averages from specific populations — not biological baselines. A baby who deviates from the chart is not biologically malfunctioning. It is expressing the natural variation within a flexible system. Ball's work has informed NHS guidance and won the Queen's Anniversary Prize for its research impact. It is not a fringe position. It is what the evidence from the best available UK research actually shows.
Sleep is one of the oldest and most successful biological strategies on earth. It has been running across hundreds of millions of years, in every shape the situation has demanded — from the hummingbird's metabolic near-shutdown to the harvest farmer's interrupted night to the parent feeding a baby at 3am. The body has never needed a rule about how to do it. It has always known.
The anxiety that has grown up around sleep in the last few decades is the newest thing in the story — younger than the factory, younger than the gas lamp, younger than the industrial norm it was built to enforce. It arrived late. It arrived with commercial interests. And it landed hardest on the people least equipped to push back: parents of young infants, already exhausted, already uncertain, looking at a waking baby and wondering what they have done wrong.
The answer the research gives to that question is consistent across the biology, the history, and the cross-cultural evidence. The baby is doing what babies do. The parent is doing what parents have always done. The norm they are both being measured against is the thing that doesn't fit.
Parents who understand the full picture — where the standard came from, what the biology actually shows, how sleep has looked across human history and across nature — are better placed to carry the genuine tiredness of early parenthood without the manufactured anxiety on top of it. That is what this piece is for.
Topics: #InOtherWords #sleepbiology #sleephistory #eighthours #sleepnorm #adaptiveinactivity #infantsleep #parentalexhaustion #sleepanxiety #harvestmoon #huntergatherer #HelenBall #YoungFamilyLife #sleepculture #iow
Why Do Animals Sleep at All? — The HWTK piece on the energy-saving argument: why sleep exists, what the animal world shows, and why the body's flexibility is a feature rather than a fault.
Why the Harvest Moon Was a Hard Day's Night — The history of all-night harvesting in plain language: what pre-industrial farming communities did with sleep, and what it tells us about the body's real flexibility.
Why Babies Don't Sleep Through the Night — and Why That's Not a Problem — The HWTK piece on infant sleep biology: why babies wake frequently, what the developmental logic is, and why the expectation that they shouldn't is more cultural than biological.
Natural Healing — The YFL essay on how the body manages stress and recovers under load — companion reading for anyone thinking about what the body is doing during months of disrupted nights.
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