New research published in PLOS ONE has found that people working outside of traditional 9–5 hours may be at increased risk of cognitive impairment.
In people exposed to night shift work in their current job, 79% had higher rates of cognitive impairment, while those who’d worked the night shift in their longest job saw 53% higher rates of cognitive impairment than daytime workers.
Researchers found that cognitive impairment was 4.83 times higher in non-White workers, people with depression, and those with high school or some college education.
New research published in Plos One has found that shift workers—anyone working outside of traditional 9–5 hours—may be at increased risk of cognitive impairment.
The study’s authors looked at how shift work—whether it be evening work or on-call and rotating shifts—affects cognitive impairment, noting that previous research has shown that shift work can impact human health. As people age, cognitive impairment—which the authors say “indicates the transitional phase between normal cognitive function and dementia”—affects independence, quality of life, and health.
For example, the study’s authors cite research indicating that night and rotating shift work were both associated with an increased risk of coronary events and peptic ulcers, while night shift work was found to be associated with type 2 diabetes. Other research cited shows a link between rotating shift work and delayed menopause. More so, the researchers say, the International Agency for Research on Cancer has classified night shift work as a probable human carcinogen.
The authors also state that while cognitive decline was historically believed to be an inevitable feature of the “normal aging process,” new evidence shows that cognitive function is modifiable across one’s lifespan—and that there are specific risk factors that affect it. These variables, they note, include lower education levels, smoking, and lack of social support among middle-aged and older adults.
The study and its findings
To assess how shift work affects cognitive function, the authors analyzed data from 47,811 middle-aged and older adults (aged 45–85 years) taken from the Canadian Longitudinal Study on Aging database: a study following 50,000 people for at least 20 years that collects biological, medical, psychological, social, lifestyle, and economic data on each individual.
From the data, the average individual’s age was 59.7, and 51.4% were females. Most participants (95%) were White, with more than half living with partners and having completed either high school or some college. More than half were former smokers and weekly drinkers, currently working, earning over 50,000 CAD as a household unit.
Based on these data, researchers looked at whether subjects had ever been exposed to shift work, whether they currently are, or whether they were exposed to shift work in their longest job. The researchers used four tests to assess participants’ cognitive function: the immediate and delayed Rey Auditory Verbal Learning Tests to assess memory and Animal Fluency and Mental Alteration to assess executive function.
Test scores took into account the subjects’ age, sex, education, and language (both French and English) and were then compared to normative data. The researchers then looked at both unadjusted and adjusted multivariable logistic regression models to determine associations between shift work variables and cognitive impairment, both for overall cognitive impairment and with regard to memory and executive function on an individual level.
The researchers found that one in five (or 21%) of individuals were exposed to some kind of shift work at some point over the course of their careers. Compared to daytime workers, people exposed to night shift work in their current jobs were found to experience higher rates (79%) of cognitive impairment. In those exposed to shift work in their longest job, 53% experienced cognitive impairment.
The researchers found that night shift work was associated with impairment in memory function and that rotating shift work was linked with impairment in executive function.
The researchers also found that overall cognitive impairment was 4.83 times higher among non-White workers, 1.8 times higher in workers with depression, and 2.37 times higher in those with high school or some college education. Overall cognitive impairment was also higher in smokers, people without a partner, and those with multi-morbidity (two or more chronic illnesses).
Overall cognitive impairment was lower in people over 55, with higher income (over 50,000 CAD as a household unit), and in those with social support.
How does shift work impact health?
According to the study’s lead author, Durdana Khan, York Faculty of Health PhD and a trainee with the York Centre for Aging Research and Education, little is known about the physiological pathways underlying shift work–related disease processes. Khan notes, however, that “several mechanisms have been hypothesized, including circadian misalignment due to disturbed sleep and light-induced suppression of melatonin levels at night.”
Because the body produces endogenous melatonin—which is low during daylight hours and high during the night—exposure to bright light at night can reduce melatonin levels, which, in turn, can lead to chronic illness, the authors posit. This is especially the case with shift workers who experience ebbing and flowing melatonin levels.
Another critical factor?: lifestyle, the authors say. “This includes eating at irregular timings, lower physical activity levels, and higher incidence of smoking and intake of alcohol,” they write. All of these behaviors, they say, create a feedback loop of desynchronized rhythms and altered metabolic cycles.
“However, additional studies are needed to confirm the association between SW [shift work] and cognitive impairment as well as any physiological pathways that underlie the mechanism,” the study notes.
Khan says that healthcare providers should “advise shift workers [patients] to get adequate sleep and maintain a healthy diet” in order to mitigate the harm that shift work can cause.
Equally important, Khan says, “It is also recommended that future occupational health policies focus on promoting regular cognitive function monitoring among shift workers to detect any early signs of cognitive function decline.”