The hidden cost of the night shift and how to sleep it off
To understand what shift work does to the body, it's worth looking at what emerging research suggests about sleep itself. Sleep does far more than give the brain and body a rest.
When we are asleep, our brain consolidates the memories of the day, processes emotions, and solves problems that defied it in the waking hours. It also strengthens immune defences and repairs muscle tissue.
Prof Russell Foster is a sleep scientist at Oxford University, who has spent a career studying the biology of the sleeping brain.
"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."
In that light, the strain of shift work becomes easier to see: it's not solely about being tired, but potentially about repeatedly disrupting a system that's doing far more behind the scenes than many people realise.
One of the most remarkable discoveries of recent years is that while we sleep, the brain cleans itself. Deep within the grey matter is plumbing called the glymphatic system. Fluid runs along tiny channels beside the brain's blood vessels, washing away the waste products that accumulate during waking hours.
So, what happens to these toxins when sleep is disrupted?
Prof Hugh Markus, a neurologist who leads the stroke medicine group at the University of Cambridge, has begun to answer this question.
Markus and a medical student, Yutong Chen, analysed the brain scans of more than 40,000 people drawn from a vast database of health records and medical scans built up over more than a decade at the UK Biobank. All of them were healthy when their scans were taken.
The researchers found they could identify those whose drainage systems were struggling. Critically though, they discovered that those with the most impaired drainage systems were significantly more likely to go on to develop dementia years later, according to Markus.
"Disruption of that flow," he says, "was playing an important role in predicting who would get dementia, in large numbers of people in the normal population."
Among the waste products the system clears are proteins called amyloid and tau, the deposits that accumulate in the brains of people with Alzheimer's disease. A single sleepless night measurably raises amyloid levels in the fluid surrounding the brain. Do that repeatedly, year after year, and the implications are troubling.
A Swedish study by researchers at the Karolinska Institute, tracking more than 13,000 shift workers, including night shift workers, for up to 41 years, found that shift work in mid-life was associated with a 36% higher risk of dementia - with the risk rising the longer someone had worked shifts.
Foster is careful not to overstate the link. "You wouldn't say poor sleep causes dementia," he says, "but if you're vulnerable, it's a potential risk factor."
Markus's data shows a possible link, but he cautions that it is a hypothesis at this stage and there are likely to be many other factors at play.
"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."
There are also tentative but growing indications of how sleep disturbance might increase the risk of heart disease. An analysis of 35 studies published last year found that sleep reduced to around 4.5 hours for three or more nights significantly raised the activity of the body's immune system. This is normally a good thing when it is roused to fight infection but also causes inflammation in the body which if persistent is associated with heart disease.
Disrupted sleep raises the stress hormone, cortisol, which in turn promotes insulin resistance and pushes the body toward a diabetic state. Higher levels of cortisol also worsens sleep further, locking workers into a self-reinforcing cycle. Add to this the sugar-hit snacking that keeps some shift workers going overnight and it makes for an extremely unhealthy cocktail.
As if that were not enough, the World Health Organization's International Agency for Research on Cancer (IARC), has classified night shift work as "probably carcinogenic to humans" and put it in the same risk group as red meat, citing evidence for links to breast, prostate, colon and colorectal cancers.
This may be because disruption to the body's circadian system alters timing of the production of melatonin, a hormone thought to have tumour-suppressing properties, as well as reduced vitamin D from lack of daylight, and the chronic low-level inflammation that broken sleep promotes.
Ekirch believes the old rhythm has never entirely died. "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 supports the biological claim: in a famous experiment, the American psychiatrist Thomas Wehr gave volunteers 14 hours of darkness approximating a pre-industrial winter night and within weeks, without any instruction, they drifted naturally into sleeping in two halves.
"The default," says Foster, "is almost certainly not a single block."
What struck Moen most forcefully was not the two-sleep pattern itself but the dearth of evidence around it. She investigated the data to assess how prevalent biphasic sleep was among shift workers, what health outcomes were associated with it, and whether a split sleep was better or worse than one exhausted block. She found almost nothing. "So I thought that's really interesting. I'll go and look properly," she says.
Looking properly has involved going through 11,000 summaries of scientific papers and working through the evidence on biphasic sleep across health, performance, and the subjective experience of shift workers. Her full results are expected later this year.
So far, Moen has found that existing research on the topic is fragmented. Some studies treat biphasic sleep as one long sleep plus a brief nap; others count only two equal periods; there is no agreed definition.
Several studies show that napping during shift work, where possible, is associated with reduced sleepiness and improved alertness. Studies of healthcare workers suggest that even a 20-to-50-minute nap during or after a shift improves focus and reduces drowsy driving on the way home.
The questions Moen's research will try to answer, more rigorously than has been attempted before, are: how common it is, what forms it takes and whether there is evidence that splitting sleep could improve health, performance, fatigue or safety.
Her husband, a shift worker himself, illustrates the problem with quiet precision. "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 like him, is permission, backed by science, 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," Moen says, "I think it's important to see how we can help them make better choices."
Top picture credit: Getty Images
Read the full story at BBC ↗
Sleep is not merely rest but a biological process during which the brain consolidates memories, regulates emotions, strengthens immunity, and clears metabolic waste through the glymphatic system. Shift work, particularly night shifts, repeatedly disrupts this system. Research links such disruption to measurable changes in brain waste clearance and to increased risks of dementia, cardiovascular disease, and certain cancers. A Swedish study tracking over 13,000 shift workers for decades found those working shifts in mid-life had a 36% elevated dementia risk. The mechanisms appear to involve disrupted circadian rhythms, altered melatonin production, elevated stress hormones, and chronic inflammation. However, researchers caution that sleep disruption is one of multiple contributing factors alongside blood pressure, smoking, and diabetes. Current evidence on split or biphasic sleep patterns among shift workers is fragmented and lacks standardised definitions. Some studies indicate that brief naps during shifts—as short as 20–50 minutes—correlate with improved alertness and reduced drowsy driving. Ongoing research aims to clarify how common biphasic sleep is among shift workers and whether supporting natural sleep patterns could improve outcomes.
Read the full story at BBC ↗
To understand what shift work does to the body, it's worth looking at what emerging research suggests about sleep itself. Sleep does far more than give the brain and body a rest.
When we are asleep, our brain consolidates the memories of the day, processes emotions, and solves problems that defied it in the waking hours. It also strengthens immune defences and repairs muscle tissue.
Prof Russell Foster is a sleep scientist at Oxford University, who has spent a career studying the biology of the sleeping brain.
"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."
In that light, the strain of shift work becomes easier to see: it's not solely about being tired, but potentially about repeatedly disrupting a system that's doing far more behind the scenes than many people realise.
One of the most remarkable discoveries of recent years is that while we sleep, the brain cleans itself. Deep within the grey matter is plumbing called the glymphatic system. Fluid runs along tiny channels beside the brain's blood vessels, washing away the waste products that accumulate during waking hours.
So, what happens to these toxins when sleep is disrupted?
Prof Hugh Markus, a neurologist who leads the stroke medicine group at the University of Cambridge, has begun to answer this question.
Markus and a medical student, Yutong Chen, analysed the brain scans of more than 40,000 people drawn from a vast database of health records and medical scans built up over more than a decade at the UK Biobank. All of them were healthy when their scans were taken.
The researchers found they could identify those whose drainage systems were struggling. Critically though, they discovered that those with the most impaired drainage systems were significantly more likely to go on to develop dementia years later, according to Markus.
"Disruption of that flow," he says, "was playing an important role in predicting who would get dementia, in large numbers of people in the normal population."
Among the waste products the system clears are proteins called amyloid and tau, the deposits that accumulate in the brains of people with Alzheimer's disease. A single sleepless night measurably raises amyloid levels in the fluid surrounding the brain. Do that repeatedly, year after year, and the implications are troubling.
A Swedish study by researchers at the Karolinska Institute, tracking more than 13,000 shift workers, including night shift workers, for up to 41 years, found that shift work in mid-life was associated with a 36% higher risk of dementia - with the risk rising the longer someone had worked shifts.
Foster is careful not to overstate the link. "You wouldn't say poor sleep causes dementia," he says, "but if you're vulnerable, it's a potential risk factor."
Markus's data shows a possible link, but he cautions that it is a hypothesis at this stage and there are likely to be many other factors at play.
"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."
There are also tentative but growing indications of how sleep disturbance might increase the risk of heart disease. An analysis of 35 studies published last year found that sleep reduced to around 4.5 hours for three or more nights significantly raised the activity of the body's immune system. This is normally a good thing when it is roused to fight infection but also causes inflammation in the body which if persistent is associated with heart disease.
Disrupted sleep raises the stress hormone, cortisol, which in turn promotes insulin resistance and pushes the body toward a diabetic state. Higher levels of cortisol also worsens sleep further, locking workers into a self-reinforcing cycle. Add to this the sugar-hit snacking that keeps some shift workers going overnight and it makes for an extremely unhealthy cocktail.
As if that were not enough, the World Health Organization's International Agency for Research on Cancer (IARC), has classified night shift work as "probably carcinogenic to humans" and put it in the same risk group as red meat, citing evidence for links to breast, prostate, colon and colorectal cancers.
This may be because disruption to the body's circadian system alters timing of the production of melatonin, a hormone thought to have tumour-suppressing properties, as well as reduced vitamin D from lack of daylight, and the chronic low-level inflammation that broken sleep promotes.
Ekirch believes the old rhythm has never entirely died. "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 supports the biological claim: in a famous experiment, the American psychiatrist Thomas Wehr gave volunteers 14 hours of darkness approximating a pre-industrial winter night and within weeks, without any instruction, they drifted naturally into sleeping in two halves.
"The default," says Foster, "is almost certainly not a single block."
What struck Moen most forcefully was not the two-sleep pattern itself but the dearth of evidence around it. She investigated the data to assess how prevalent biphasic sleep was among shift workers, what health outcomes were associated with it, and whether a split sleep was better or worse than one exhausted block. She found almost nothing. "So I thought that's really interesting. I'll go and look properly," she says.
Looking properly has involved going through 11,000 summaries of scientific papers and working through the evidence on biphasic sleep across health, performance, and the subjective experience of shift workers. Her full results are expected later this year.
So far, Moen has found that existing research on the topic is fragmented. Some studies treat biphasic sleep as one long sleep plus a brief nap; others count only two equal periods; there is no agreed definition.
Several studies show that napping during shift work, where possible, is associated with reduced sleepiness and improved alertness. Studies of healthcare workers suggest that even a 20-to-50-minute nap during or after a shift improves focus and reduces drowsy driving on the way home.
The questions Moen's research will try to answer, more rigorously than has been attempted before, are: how common it is, what forms it takes and whether there is evidence that splitting sleep could improve health, performance, fatigue or safety.
Her husband, a shift worker himself, illustrates the problem with quiet precision. "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 like him, is permission, backed by science, 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," Moen says, "I think it's important to see how we can help them make better choices."
Top picture credit: Getty Images
Read the full story at BBC ↗
Sleep consolidates memories, processes emotions, strengthens immune defences, and repairs muscle tissue. The brain's glyphatic system clears waste products during sleep via fluid flowing through channels beside blood vessels. A Cambridge study of over 40,000 people found those with the most impaired brain drainage systems were significantly more likely to develop dementia years later. Amyloid and tau proteins, which accumulate in Alzheimer's disease, are among the waste products cleared during sleep. A single sleepless night measurably raises amyloid levels in the fluid surrounding the brain. A Swedish study tracking 13,000+ shift workers for up to 41 years found shift work in mid-life was associated with 36% higher dementia risk. Poor sleep causes dementia. Sleep disruption is a potential risk factor for dementia in vulnerable individuals, but other factors such as blood pressure, smoking, and diabetes likely play larger roles in Alzheimer's risk. An analysis of 35 studies found that sleep reduced to 4.5 hours for three or more nights significantly raised immune system activity, which can cause persistent inflammation linked to heart disease. Disrupted sleep raises cortisol, promoting insulin resistance and creating a self-reinforcing cycle of poor sleep. The WHO's IARC classifies night shift work as 'probably carcinogenic to humans' in the same risk group as red meat. Night shift work increases cancer risk primarily due to circadian disruption affecting melatonin, reduced vitamin D, and chronic inflammation. Middle-of-the-night insomnia may represent a relic of pre-industrial biphasic sleep patterns rather than a disorder. Research on biphasic sleep among shift workers is fragmented with no agreed definitions of what constitutes biphasic sleep. Several studies show that napping during shift work, where possible, is associated with reduced sleepiness and improved alertness. A 20-to-50-minute nap during or after a shift improves focus and reduces drowsy driving among healthcare workers. The body may have a default biphasic sleep pattern rather than a single sleep block.
Read the full story at BBC ↗
- Sleep performs critical biological functions including memory consolidation, immune defence, and brain waste clearance via the glymphatic system
- Disrupted sleep from shift work is associated with impaired brain drainage, higher dementia risk (36% in one Swedish study of 13,000+ workers), and increased cardiovascular and cancer risk
- Research on biphasic or split sleep patterns among shift workers remains fragmented with no agreed definitions; some studies suggest strategic napping (20–50 minutes) may improve alertness and safety
- The WHO's IARC classifies night shift work as 'probably carcinogenic' due to circadian disruption affecting melatonin production and chronic inflammation
- Evidence suggests the body may have a natural inclination toward biphasic sleep; ongoing research aims to determine whether supporting this pattern could improve shift workers' health outcomes