It is four o’clock in the morning on a hospital ward. The lights are dim, the corridors still, and a resident doctor is nine hours into a shift that will end just as the Sun begins to rise. Her legs ache, her eyes burn, and when she eventually walks through her own front door, sleep refuses to come. Her body, locked into rhythms forged by millions of years of evolution, insists the day has begun.
This is not weakness. It is biology fighting back against the demands of modern life, and it is a battle being waged silently by millions. Nurses, paramedics, lorry drivers, factory workers, engineers, the army of overnight labour that keeps Britain moving while everyone else dreams. They pay for that work with their health, and new research is finally spelling out the price.
When the Body’s Drainage System Fails
Sleep is far more than rest. While we are unconscious, the brain consolidates memories, processes emotions, solves problems, strengthens the immune system and repairs muscle. It also performs a task once thought impossible: it cleans itself.
Deep within the grey matter, a network of tiny channels known as the glymphatic system pumps fluid alongside blood vessels, flushing out the metabolic debris of the waking day. The waste products include amyloid and tau, the same proteins that clump into the plaques and tangles of Alzheimer’s disease.
Prof Hugh Markus, who leads the stroke medicine group at the University of Cambridge, has been examining what happens when this drainage fails. Working with medical student Yutong Chen, the pair analysed brain scans from more than 40,000 healthy volunteers held in the UK Biobank, a vast repository of medical imaging built up over more than a decade. They identified participants whose glymphatic systems appeared sluggish, then tracked what happened to them in the years that followed.
The result was striking. Those with the most impaired drainage were significantly more likely to go on to develop dementia.
“Disruption of that flow was playing an important role in predicting who would get dementia, in large numbers of people in the normal population,” Markus says.
A single sleepless night measurably raises amyloid levels in the fluid surrounding the brain. Repeated over years, the consequences may be severe. A Swedish study from the Karolinska Institute tracked more than 13,000 shift workers for up to 41 years and found that mid-life shift work was associated with a 36% higher risk of dementia, with risk climbing alongside years on the rota.
Prof Russell Foster, a sleep scientist at Oxford University, is cautious about drawing a direct line. “You wouldn’t say poor sleep causes dementia,” he says, “but if you’re vulnerable, it’s a potential risk factor.”
Markus agrees that sleep is only part of the picture. “Sleep matters,” he says, “but so do the big vascular things: blood pressure, smoking, diabetes. What’s never mentioned is how much of the risk of Alzheimer’s comes from those, things we could actually do something about.”
The Heavy Toll on Heart, Mind and Metabolism
The damage does not stop at the brain. An analysis of 35 studies published last year found that restricting sleep to around 4.5 hours for three or more nights significantly ramped up immune activity, which, while useful against infection, also drives inflammation linked to heart disease.
Disrupted sleep raises cortisol, the body’s chief stress hormone. Cortisol promotes insulin resistance, pushing workers towards a pre-diabetic state, and high cortisol also makes sleep worse, creating a self-reinforcing spiral that is difficult to break. Late-night sugary snacks, the fuel of choice for many overnight workers, make matters worse still.
The World Health Organization’s International Agency for Research on Cancer has classified night shift work as “probably carcinogenic to humans”, placing it in the same risk group as red meat. The mechanisms are still being untangled, but likely contributors include suppression of melatonin, a hormone thought to suppress tumours; reduced vitamin D from lack of daylight; and the chronic, low-grade inflammation that broken sleep promotes.
Foster, who has spent his career studying the sleeping brain, is blunt about what shift work represents. “Sleep is a pillar of our health,” he says, “in the same way we think about diet and exercise. We have to take control of it.”
An Echo from the Pre-Industrial Night
For more than three million people in the UK alone, controlling sleep is a luxury. Their work dictates when they rest, and their bodies resist every attempt at override.
But researchers now think help may be at hand, and from an unexpected quarter. Dr Line Victoria Moen, of Norway’s National Institute of Occupational Health in Oslo, has been studying shift workers in the far north of Norway, above the Arctic Circle, where summer brings perpetual daylight and sleep must be fought for against the Sun itself.
The workers wore Oura Rings to track their rest. As Moen pored over the data, a clear pattern emerged. Rather than collapsing into one long, exhausted sleep on returning home, many were splitting their rest into two distinct blocks: roughly nine until one, then again in the late afternoon before the next shift.
“Their body is forcing them to wake up,” she says, “and recover some additional sleep in the afternoon.”
This pattern has a name, biphasic sleep, two separate periods of rest within 24 hours, and it is far from a modern invention. Roger Ekirch, a historian at Virginia Tech, has spent four decades combing the archives of pre-industrial Europe. His findings are unambiguous: until the 19th century, humans overwhelmingly slept in two halves.
A typical family, Ekirch found, would retire around nine, wake naturally after midnight for prayer, chores or quiet conversation, then drift back into what they called their “second sleep”. The pattern only began to fade as artificial lighting spread. Gas lamps illuminated London’s streets from 1807, people stayed up later, and the gap between the two sleeps narrowed and eventually closed.
Ekirch believes the old rhythm has never truly vanished. “Middle-of-the-night insomnia is the most prevalent sleep disorder in many countries,” he says. “And I’d argue that, in many cases, it isn’t a disorder at all. It instead represents a persistent echo, a relic of this earlier pattern of sleep.”
Foster’s own laboratory work backs the biology. In a landmark experiment, the American psychiatrist Thomas Wehr confined volunteers to 14 hours of darkness, mimicking a pre-industrial winter night, and within weeks, without instruction, they drifted naturally into two-block sleep. “The default,” Foster says, “is almost certainly not a single block.”
Permission to Nap
What struck Moen most was not the pattern itself, but the absence of rigorous evidence around it. She reviewed more than 11,000 summaries of scientific papers. The research on biphasic sleep, she found, was fragmented, poorly defined and woefully incomplete. Some studies treated it as a long sleep plus a brief nap; others insisted on two roughly equal blocks. There is no agreed definition.
Her husband, himself a shift worker, illustrates the gap between lived experience and clinical guidance. “He always wakes very early, after only three or four hours,” she says. “There’s no-one home, it’s dark, and still, he can’t sleep. His day rhythm drags him up.” His body will not be overruled by a blackout blind.
What Moen wants to give him, and the millions in similar circumstances, is permission. Permission, backed by evidence, to stop fighting the body’s signal and work with it instead.
“Since we know that many shift workers can’t really avoid sleeping during the day,” she says, “I think it’s important to see how we can help them make better choices.”
Existing evidence is encouraging, if patchy. Studies of healthcare workers suggest that a 20-to-50-minute nap during or after a shift sharpens focus and reduces the risk of drowsy driving on the way home. Moen’s full results are expected later this year, and she hopes they will form the basis of clear clinical guidelines.
“They get very anxious because they can’t sleep long after a night shift,” she says. “It would be nice to be able to tell them that, actually, a good nap in the afternoon will help.”
Why it Matters
The hidden cost of the night shift is not just fatigue, it is a measurable, accumulating assault on the brain, the heart and the body’s defences, borne disproportionately by the workers who hold modern society together. If the science continues to point towards biphasic sleep as a more natural, more sustainable pattern for those who must work through the night, then clinical guidance, workplace policies and personal habits may all need to be rewritten. Millions of people are fighting a battle they were never equipped to win. It is time the science gave them a way to stop losing.
