It’s four in the morning, and the ward is still. A resident doctor, on her feet for the past nine hours, is feeling exhausted. Her muscles ache, and her eyes are struggling to stay open. Yet, when her shift wraps up at six and she finally gets home, sleep is elusive.
Her body’s internal clock, calibrated over eons to sync with the Sun’s natural cycles, insists it’s time to be awake. Darkness, earplugs, and blackout blinds can’t quite drown it out.
This isn’t simply a personal issue. It highlights a significant clash between the demands of her job and fundamental human biology, affecting countless shift workers. This group includes nurses, paramedics, engineers, truck drivers, and factory employees—those who ensure society functions while others rest.
The mounting scientific evidence about this ongoing struggle against our internal rhythms and the impact on health—such as heart disease, strokes, cancer, mental health issues, and the potential for memory loss—is increasingly hard to overlook.
Scientists are now investigating if altering sleep patterns might help reduce the negative outcomes associated with night shifts. Some studies are even looking into a surprising strategy: splitting sleep into two shorter blocks instead of trying for one long period during the day. This may prove to be more effective for night workers.
The toll of shift work
To grasp the effects of shift work on the body, we need to consider what recent research suggests about sleep. Sleep does a lot more than just provide rest.
When we sleep, our brain solidifies daily memories, processes feelings, and addresses problems faced during the day. It also bolsters our immune system and repairs muscle tissue.
Professor Russell Foster, a sleep scientist at Oxford, emphasizes that “Sleep is a cornerstone of our health,” akin to diet and exercise. “We need to take charge of it.”
Thus, the challenges of shift work become clearer: it’s not just about fatigue but also about repeatedly interrupting a crucial system that does much more than most people are aware of.
A notable finding in recent years is that during sleep, the brain cleanses itself. The glymphatic system circulates fluid along channels in the brain, helping to wash away accumulated waste products from our waking hours.
But what happens to these toxins when sleep is disrupted?
Professor Hugh Markus, head of the stroke medicine group at the University of Cambridge, is exploring this question.
Alongside a medical student, Yutong Chen, Markus analyzed brain scans from over 40,000 individuals pulled from a massive health database at the UK Biobank. All participants were healthy when scanned.
The study uncovered that they could pinpoint those with impaired waste removal systems. Alarmingly, those with significant drainage issues were more likely to develop dementia later on.
“Disruption of that flow,” Markus explains, “was crucial in predicting dementia within a large segment of the general population.”
Among the waste cleared by this system are proteins called amyloid and tau, which build up in the brains of Alzheimer’s patients. Even a single night of poor sleep can spike amyloid levels in the surrounding brain fluid. Repeatedly doing this over the years leads to worrying implications.
A Swedish study from the Karolinska Institute took over 13,000 shift workers and tracked them for up to 41 years, finding that mid-life shift work is linked to a 36% increase in dementia risk, escalating with longer durations in shift work.
Foster is careful not to exaggerate the connection. “You couldn’t claim that poor sleep causes dementia,” he clarifies, “but for those at risk, it’s a potential concern.”
Markus’s findings indicate a possible relationship, but he points out that this is still a theory, with many other factors needing consideration.
“Sleep is important,” he states, “but so are major vascular issues like blood pressure, smoking, diabetes. It’s often overlooked how much Alzheimer’s risk could be tied to those factors—things we can actually address.”
There are also emerging indications of how disturbed sleep could raise heart disease risks. An analysis of 35 studies published last year indicated that restricting sleep to about 4.5 hours over three nights ramped up immune system activity. This is beneficial for fighting infections, but enduring inflammation is linked to heart issues.
Sleep disruption raises cortisol, the stress hormone, which promotes insulin resistance and pushes the body towards diabetes. Higher cortisol levels further disturb sleep, creating a negative feedback loop. Adding the unhealthy late-night snacks that some shift workers rely on makes the situation even worse.
Additionally, the World Health Organization’s International Agency for Research on Cancer has classified night shift work as “probably carcinogenic,” aligning it with risks associated with red meat due to associations with breast, prostate, colon, and colorectal cancers.
This connection may arise from disruptions to circadian rhythms, influencing melatonin production—hormones believed to suppress tumors—as well as decreased vitamin D levels from insufficient sunlight, and the ongoing low-level inflammation from poor sleep.
Biphasic sleep
For over three million UK night shift workers, this might sound alarming. However, researchers are starting to identify potential solutions.
Dr. Line Victoria Moen, a researcher at Norway’s National Institute of Occupational Health, is examining whether planned naps can help.
The research team encountered intriguing results while studying shift workers in the Arctic Circle, where sunlight hardly fades in summer and sleep can be a challenge. This unique setting provided a great opportunity to observe the struggle between the body’s clock and the environment.
Moen tracked shift workers in northern Norway. As they wore Oura Rings to monitor their sleep patterns, a distinct trend emerged. Many didn’t simply dive into long sleeps after their shifts. Instead, they were getting two separate sleep periods—one from nine to one, and another in the afternoon before working again.
“Their bodies are making them wake up,” she observed, “and recover with that extra sleep in the afternoon.”
This pattern, known as biphasic sleep, consists of two sleep chunks rather than one. It resonates with historical accounts of sleep patterns prior to industrial times when humans often slept in two phases. Researchers suggest this might be a more natural rhythm, even though the reasons are still being debated.
Historian Roger Ekirch, who has spent decades in archives examining pre-industrial Europe, found that this two-phase sleep was common in the Western world until the 19th century.
Typical families would sleep around nine, wake naturally post-midnight for various activities like prayers or chats, and then return to what they termed their “second sleep.”
This pattern was prevalent until the advent of artificial light, which altered nighttime habits—gas lamps began lighting London’s streets in 1807, encouraging people to stay up later, shrinking the duration of these two sleeps until they effectively merged.
Ekirch believes this old rhythm hasn’t completely faded. “Middle-of-the-night insomnia is the leading sleep disorder in many areas,” he notes.
“I’d argue, in many instances, it isn’t strictly a disorder. It’s more of a lingering reminder of an older sleep pattern.”
Supporting this biological claim, Foster’s lab work aligns with findings from a well-known study where American psychiatrist Thomas Wehr placed volunteers under 14 hours of darkness, replicating a pre-industrial winter night. Within weeks, participants began sleeping in two segments without any guidance.
“The default,” Foster concludes, “is almost certainly not one continuous block.”
What struck Moen most was how little concrete evidence there was on biphasic sleeping. She scrutinized the data to determine its prevalence among shift workers, its health impacts, and whether split sleep is more effective than a single long sleep. Her findings so far reveal limited research, prompting her to dig deeper.
Her investigation has required reviewing 11,000 scientific summaries, focusing on the evidence surrounding biphasic sleep in regard to health, performance, and shift workers’ experiences. Comprehensive results are expected later this year.
Initially, Moen found that research on the topic was scattered—some studies considered biphasic sleep as one long phase followed by a nap, while others categorized it into two equal periods, and there was no consistent definition.
Sleep anxiety
Ultimately, Moen hopes her research will yield clear clinical guidelines to assist shift workers and others grappling with disrupted sleep.
“They often feel anxious because they struggle to sleep well after a night shift,” Moen explains. “It would be wonderful to assure them that a short afternoon nap could help.”
Numerous studies indicate that napping during shifts can reduce fatigue and enhance alertness. Research on healthcare professionals suggests that even a brief nap of 20 to 50 minutes following a shift can improve attention and lessen drowsiness during the drive home.
The rigorous inquiry Moen intends to pursue will cover questions about how common biphasic sleeping is, its characteristics, and whether it can enhance health, performance, fatigue, or safety.
Her husband, a shift worker himself, exemplifies the issue with quiet clarity. “He frequently wakes up early after only three or four hours,” she points out. “It’s still dark and no one’s around, yet he can’t fall back asleep. His body clock insists it’s time to be up.”
His biological rhythms won’t yield to blackout curtains. What Moen aspires to provide him—and the millions like him—is the encouragement, grounded in science, to embrace these signals rather than resist them.
“Considering that many shift workers can’t avoid sleeping during the day,” she reflects, “it’s vital to figure out how to assist them in making informed choices.”
