Shift Work Linked to Slower Performance on a Common Cognitive Screening Test, Study Finds

Summary: Researchers report that people who currently work shifts, or who worked shifts within the past five years, require more time to complete a commonly used cognitive screening task than those who have never worked shifts or who quit shift work more than five years ago.

Source: Uppsala University.

Overview: A study led by researchers at Uppsala University compared performance on a widely used cognitive screening task between shift workers and non-shift workers. The analysis, based on nearly 7,000 participants in the Swedish EpiHealth cohort, found that current shift workers and those who stopped shift work within the last five years took longer to complete the Trail Making Test, a timed assessment often used to screen for cognitive impairment. Individuals who had ended shift work more than five years earlier performed similarly to people who never worked shifts. The findings are published in the journal Neurobiology of Aging.

The research team, including scientists from Uppsala University and Malmö University, analyzed data from 7,143 middle-aged and older adults (ages 45–75) who participated in EpiHealth. The principal outcome measure was the Trail Making Test (TMT), a standard neuropsychological test consisting of two parts. In Part A, participants draw a line to connect numbered circles in ascending order (1 to 25). Part B is more demanding: participants must alternate between numbers and letters in ascending sequence (for example, 1–A–2–B, and so on). Completion time on both parts of the TMT is commonly used as an indicator of processing speed, visual scanning, and some aspects of executive function, and is known to typically increase with age.

After adjusting for relevant confounding factors such as age, education level, and average sleep duration, the investigators observed that people currently engaged in shift work and those who had stopped shift work within the past five years required more time to finish the TMT than non-shift workers. In contrast, participants who had ceased shift work more than five years earlier performed on par with non-shift workers. These patterns suggest that the negative association between shift work and TMT performance may be reversible, but that recovery in test-related cognitive functions could take several years following the end of shift work.

“Our results indicate that shift work is linked to poorer performance on a test that is frequently used to screen for cognitive impairment in humans,” says Christian Benedict, associate professor in the Department of Neuroscience at Uppsala University and corresponding author of the study. The poorer performance was observed only among current shift workers and those who had worked shifts during the previous five years; it was not seen among people who had been away from shift work for more than five years.

Study details and interpretation

The investigators used self-reported shift work history to classify participants as current shift workers, recent former shift workers (stopped within the last five years), or past shift workers (stopped more than five years ago). The primary measure was time to complete the Trail Making Test after accounting for potential confounders. The observed association between recent or ongoing shift work and slower TMT completion times remained after statistical adjustment, indicating a robust link in this cohort of middle-aged and elderly adults.

Because the study is observational, it cannot prove a causal relationship; however, the pattern that performance normalizes after a longer period away from shift work supports the possibility that effects on test-related cognitive functions may be at least partially reversible. The authors note that it could take at least five years for recovery of the specific cognitive skills measured by the TMT among former shift workers.

Funding, authorship and publication

Funding: The work was primarily supported by the Swedish Research Council, the Swedish Brain Foundation, AFA Insurance, and the Novo Nordisk Foundation. The funding bodies had no role in study design, data collection, analysis, interpretation, manuscript writing, or the decision to publish.

Authors and source: The study reported here is credited to Olga E. Titova, Eva Lindberg, Sölve Elmståhl, Lars Lind, Helgi B. Schiöth, and Christian Benedict, with the corresponding author affiliated with Uppsala University. The findings were published online in Neurobiology of Aging.

Abstract (concise)

Shift work has been proposed to increase risk of cognitive disturbances, although evidence has been mixed. Using data from 7,143 adults aged 45–75 in the Swedish EpiHealth study, researchers examined associations between self-reported shift work history and performance on the Trail Making Test, a tool commonly used to assess executive aspects of cognition that decline with age. After adjusting for potential confounders including age, education, and sleep duration, current and recent former shift workers (those who worked shifts during the past five years) showed worse performance on the TMT compared with non-shift workers. Past shift workers who had been off shift work for more than five years did not differ from non-shift workers. These results indicate that shift work history is associated with poorer TMT performance in this cohort, and that test-related deficits may diminish after several years away from shift work.

Reference: Association between shift work history and performance on the trail making test in middle-aged and elderly humans: the EpiHealth study. Published in Neurobiology of Aging.