It is four in the morning, and the ward is quiet. A resident doctor has been on her feet for nine hours. She is tired, her muscles are sore and her eyes are straining, but when her shift ends at six in the morning and she finally gets home, she struggles to sleep. Her internal clock, built over millions of years of evolution to tune human biology to the rising and setting of the Sun, is insisting it is morning. Time to wake up. Time to be alert. No amount of darkness, earplugs or blackout blinds can entirely silence it. This is not a personal failing. It is a collision between the demands of her job and some of the deepest machinery in the human body. This is playing out, invisibly, in the lives of millions of shift workers. Among them are the nurses, paramedics, engineers, lorry drivers and factory workers, who keep the country running while everyone else sleeps. Image source, Getty Images And the scientific evidence about what this relentless battle with our own internal clocks and modern living costs them – in heart attacks, strokes, cancer, mental illness, and quite possibly their precious memories – is increasingly difficult to ignore. Now scientists are beginning to explore whether changing how we sleep can play a role in mitigating the toll of night shifts, and potentially alleviate the ill-effects of disrupted nights. Their studies are also testing a surprising theory: that splitting sleep into two separate blocks – rather than attempting to force one long stretch during the day – may in fact be the most effective sleep pattern for people working through the night.
The cost of shift work 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.” Image source, Getty Images 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
