The hidden cost of the night shift and how to sleep it off

Emerging research is shedding crucial light on the profound impact of shift work on the human body, revealing that sleep is far more than a period of rest. When we are asleep, our brains are actively engaged in vital processes: consolidating daily memories, processing emotions, and solving complex problems that elude us during waking hours. Furthermore, sleep is essential for strengthening our immune defenses and repairing muscle tissue. Professor Russell Foster, a distinguished sleep scientist at Oxford University with a career dedicated to studying the biology of the sleeping brain, emphasizes its critical role. "Sleep is a pillar of our health," he states, "in the same way we think about diet and exercise. We have to take control of it." Understanding this fundamental importance makes the strain of shift work undeniably clear; it’s not merely about experiencing fatigue, but about the potential for repeatedly disrupting a sophisticated system performing far more behind the scenes than commonly recognized.

One of the most significant discoveries in recent years is the brain’s remarkable self-cleaning mechanism that operates during sleep. Deep within the brain’s grey matter lies the glymphatic system, a sophisticated network of plumbing. Fluid circulates through minute channels adjacent to the brain’s blood vessels, effectively flushing away the waste products that accumulate during our waking hours. But what happens to these accumulated toxins when sleep is disrupted? Professor Hugh Markus, a neurologist leading the stroke medicine group at the University of Cambridge, is beginning to provide answers.

Professor Markus, in collaboration with medical student Yutong Chen, meticulously analyzed the brain scans of over 40,000 individuals sourced from the UK Biobank, a vast repository of health records and medical scans meticulously compiled over more than a decade. All participants were in good health at the time of their scans. The researchers were able to identify individuals whose glymphatic drainage systems were compromised. Crucially, their findings revealed a significant correlation: those with the most impaired drainage systems were substantially more likely to develop dementia years later, as reported by Professor Markus. "Disruption of that flow," he elaborates, "was playing an important role in predicting who would get dementia, in large numbers of people in the normal population."

Among the waste products diligently cleared by the glymphatic system are proteins such as amyloid and tau. These proteins are known to deposit in the brains of individuals with Alzheimer’s disease. Even a single night of sleep deprivation can measurably elevate amyloid levels in the cerebrospinal fluid. When this occurs repeatedly, year after year, the implications become deeply concerning. A comprehensive Swedish study conducted by researchers at the Karolinska Institute, which tracked more than 13,000 shift workers, including those on night shifts, for up to 41 years, established a link between mid-life shift work and a 36% higher risk of dementia. The study further indicated that this risk escalated with the duration of shift work. Professor Foster, while acknowledging the potential link, advocates for a nuanced perspective. "You wouldn’t say poor sleep causes dementia," he cautions, "but if you’re vulnerable, it’s a potential risk factor." Professor Markus’s data suggests a possible association, but he stresses that this remains a hypothesis at this stage, acknowledging the probable involvement of numerous other contributing factors. "Sleep matters," he asserts, "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."

Beyond the cognitive implications, there are growing, albeit tentative, indications of how sleep disturbance might elevate the risk of heart disease. An extensive analysis of 35 studies published last year revealed that reducing sleep to approximately 4.5 hours for three or more consecutive nights significantly amplified the activity of the body’s immune system. While heightened immune activity is beneficial in combating infection, persistent inflammation, which this can trigger, is strongly associated with an increased risk of heart disease.

Furthermore, disrupted sleep leads to elevated levels of the stress hormone cortisol. This, in turn, promotes insulin resistance, pushing the body towards a diabetic state. The persistent presence of higher cortisol levels also exacerbates sleep problems, trapping individuals in a detrimental, self-perpetuating cycle. Compounding these issues, some shift workers resort to high-sugar snacks to maintain alertness during overnight shifts, creating an exceptionally unhealthy combination.

Adding to this growing list of concerns, the World Health Organization’s International Agency for Research on Cancer (IARC) has classified night shift work as "probably carcinogenic to humans." This places it in the same risk category as red meat and is based on evidence linking it to an increased risk of breast, prostate, colon, and colorectal cancers. This potential carcinogenicity may stem from the disruption of the body’s circadian rhythms, which alters the timing of melatonin production – a hormone believed to possess tumor-suppressing properties. Reduced exposure to vitamin D from lack of daylight and the chronic low-level inflammation promoted by broken sleep are also considered contributing factors.

The historical context of sleep patterns offers a fascinating perspective. Roger Ekirch, a historian, suggests that the "old rhythm" of sleep has never entirely vanished. He notes, "Middle-of-the-night insomnia is the most prevalent sleep disorder in many countries. 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." This earlier pattern, often referred to as biphasic sleep, involved sleeping in two distinct periods. Professor Foster’s laboratory research provides biological support for this idea. In a notable experiment, American psychiatrist Thomas Wehr exposed volunteers to 14 hours of darkness, mimicking pre-industrial winter nights. Within weeks, without any explicit instruction, these volunteers naturally adopted a sleep pattern of two halves. "The default," Foster observes, "is almost certainly not a single block."

The research of Dr. Marieke van der Zanden, a sleep researcher, was particularly struck by the lack of scientific investigation into this biphasic sleep pattern. She embarked on an in-depth exploration of the existing data to assess its prevalence among shift workers, its associated health outcomes, and whether a split sleep pattern offered advantages over a single, prolonged, and potentially exhausting block of sleep. Her initial findings revealed a surprising paucity of information. "So I thought that’s really interesting. I’ll go and look properly," she recounts.

This thorough investigation has involved meticulously reviewing over 11,000 scientific paper summaries and synthesizing the evidence on biphasic sleep across various domains, including health, performance, and the subjective experiences of shift workers. Her comprehensive results are anticipated later this year. So far, Dr. van der Zanden has observed that current research on biphasic sleep is fragmented. Some studies define it as one long sleep coupled with a brief nap, while others count only two equal periods, indicating a lack of a universally agreed-upon definition.

Despite the definitional challenges, several studies indicate that napping during shift work, where feasible, is associated with reduced sleepiness and improved alertness. Research involving healthcare professionals suggests that even a short nap of 20 to 50 minutes during or immediately after a shift can enhance focus and decrease the risk of drowsy driving on the commute home. Dr. van der Zanden’s research aims to address the existing gaps by rigorously investigating the prevalence of biphasic sleep, its various manifestations, and whether there is scientific evidence to support its potential to improve health, performance, reduce fatigue, or enhance safety.

Her husband, who himself is a shift worker, provides a poignant illustration of the challenges. "He always wakes very early, after only three or four hours," she explains. "There’s no-one home, it’s dark, and still, he can’t sleep. His day rhythm drags him up." His body, she notes, refuses to be overridden by blackout blinds. What Dr. van der Zanden aspires to provide for him, and for millions like him, is the scientific validation to cease fighting the body’s natural signals and instead to work in harmony with them. "Since we know that many shift workers can’t really avoid sleeping during the day," she concludes, "I think it’s important to see how we can help them make better choices."