Health authorities have long been sounding the alarm about shift work, especially night shifts. In 2019, the International Agency for Research on Cancer (IARC) categorized shift work as a class 2A “probable human carcinogen.” “That’s the same group as tobacco!” exclaims Dr. Juan Antonio Madrid, Professor of Physiology and Director of the Chronobiology Laboratory at the University of Murcia in Spain. Madrid believes that there’s sufficient experimental evidence from research on animals as well as indirect evidence from people to support that claim. Moreover, he explains, the chronic alteration of the biological rhythms involved in shift work has an enormous impact on health: “It exacerbates the release of inflammatory mediators; reduces the immune system’s activity; causes metabolic effects that can lead to prediabetes, higher triglyceride levels, hypertension, and a higher risk of myocardial infarction and stroke, etc. In short, shift work aggravates a wide variety of very significant pathologies.”
Over the long term, those effects continue, even well after shift work ends. A study performed on mice, which was recently published in the scientific journal Neurobiology of Sleep and Circadian Rhythms, found that the consequences of shift work schedules in early adulthood (the equivalent of 18-24 years old in human age) persist into middle age (55 to 60 years of age), even when the rodents had returned to normal schedules during the intervening period. “We have found that exposure to shift work schedules during early adulthood exacerbates ischemic stroke outcomes in midlife, specifically in females. And even when the study subjects returned to a normal schedule, the effects on sleep-wake rhythms from early exposure to shift work cycles persisted so that in midlife these subjects were waking earlier and becoming active at the wrong time of day and night,” explained Professor David J. Earnest, the study’s principal investigator and a researcher at the Texas A&M University Center for Biological Clocks Research.
Juan Antonio Madrid says this data is “interesting” because it disproves the idea that shift work’s effects are only temporary and disappear once a more regular/normal schedule is restored. “We know that the incidence of cardiovascular disease among the shift-working population is around 20%, as compared to 7% for daytime workers. In the case of non-shift workers, the risk percentage remains at 15%. The time spent working shifts –for a long time, not if we’re talking about one or two years – takes a toll and causes changes that are not completely reversible,” the professor says. He points out that the research was conducted on mice, so extrapolating the data to humans is risky. Nevertheless, the study does “offer clues about what happens to [body] mechanisms.” The research on rodents has the advantage of eliminating confounding factors. Shift work (especially the night shift) is also associated with unhealthy lifestyle habits (being more sedentary, having a greater tendency to use tobacco and alcohol, eating a poorer diet), “which can mask and aggravate a shift worker’s health risks that aren’t solely a function of the hours worked.”
Is it possible to adapt to working at night?
Research published in the July issue of eBioMedicine (The Lancet) debunks another myth about working the night shift: that “one gets used to everything and the body can adapt to anything.” The study monitored 63 night-shift workers (who had three or more 10-hour night shifts per week) and 77 workers who alternated between morning and evening shifts; the participants were all employees at the Paul-Brousse Hospital just outside Paris. The results were clear: night shift workers had significantly impaired sleep quality and circadian rhythms, even though they’d worked the same night shift for over five years. “One of the most surprising findings was how poorly these hospital workers’ circadian systems adapted to the night shift, despite working that shift for a long time. And the effects weren’t just for the days they worked but also their off days, which indicates that some night shift workers don’t recover normal circadian function even on their days off,” explains Francis Lévi, Director of Research in Chronotherapy, Cancers and Transplantation at the University Paris-Saclay’s School of Medicine.
“It’s nearly impossible to adapt to night shift work,” avers Juan Antonio Madrid. He believes that such adaptations are only feasible in isolated environments such as oil rigs, where night shift workers can maintain a more regular schedule. “Chaotic habits are the problem, the fact that you eat at a certain time one day and then at a different time the next day, one day you exercise your body at a given time and the next day you do it at another time, one day you go to sleep at a specific time and then the following day you go to bed at a different time. Biological clocks are designed to anticipate, to prepare the body for regular and predictable events. The body isn’t prepared when we don’t provide that predictable framework,” he argues.
Despite this extreme difficulty, Madrid says that night shift workers can use three basic strategies to try to minimize shift work’s impact on rest and circadian rhythms. The first strategy is establishing a routine: “We have to regularize our eating and sleeping schedules insofar as possible; if you have to work at night, but then on the days when you’re not working, you completely change your schedule to meet family or social obligations, you’re continually altering your schedule. That’s a bad thing. It’s much more aggressive to our bodies than just sticking to a delayed sleep schedule each day, whether we’re working [that day] or not. We must ensure that we go to sleep at the same time every day, and that we get at least four hours of good sleep. The key is to establish a routine when our work schedules promote irregularity.”
The second strategy is to pursue contrast by getting enough physical activity (”because that makes you more resistant to shift work’s harmful effects“) and exposure to natural light during the day, because sunlight is a powerful biological regulator. Finally, we should try to synchronize mealtimes/fasts and sleep cycles/darkness, which is the most difficult thing to do: “Fasting should coincide with sleep/darkness, which is quite a challenge for people who work the night shift,” Madrid says.
Finally, Professor Francis Lévi emphasizes the need for night shift workers to have access to “specific medical evaluations throughout their professional lives” because they are exposed to greater health risks. “These days we’re able to objectively and non-invasively assess circadian rhythms and sleep health almost in real time; then, when necessary, [we can] design preventive measures for individual workers and evaluate their effectiveness,” he notes.