Jobs that require irregular work hours that are not 9 to 5, such as truck driving, firefighting, or law enforcement, have been associated with health complications, including heart disease, metabolic syndrome, and certain types of cancer. However, the effect shift work has on cognitive function is relatively unknown. An observational study published in the journal Occupational & Environmental Medicine suggests that shift work can disrupt our body’s internal clock, leading to impaired brain function.

"The cognitive impairment observed in the present study may have important safety consequences not only for the individuals concerned, but also for society as a whole, given the increasing number of jobs in high hazard situations that are performed at night," the research team said in a statement.

The study’s sample size included over 3,000 participants aged 32, 43, 52, and 62, who were either still working in a certain field or had retired from their job in 1996, 2001, or 2006. Researchers assessed long- and short-term memory, processing speed, and overall cognitive abilities of 1,197 patients from three occupational health doctors in the southern France. Patients had worked shifts for at least 50 days of the year and were assessed at three different time points.

One in five participants who were currently in work and 17.9 percent of retired participants worked a shift pattern that rotated between mornings, afternoons, and nights. These participants recorded lower scores on memory, processing speed, and overall cognitive function compared to those who worked normal office hours. Participants who were currently working the morning, afternoon, and night rotating shift pattern, had previously worked it, or had done it for 10 or more years suffered a cognitive decline related to 6.5 years of aging.

According to data from the Bureau of Labor Statstics, around 15 million people in the United States work full time on evening shift, night shift, rotating shift, or other irregular shifts arranged by employers. Common side effects experienced by shift workers include sleepiness and fatigue, which can lead to poor concentration, absenteeism, accidents, errors, injuries, and death in dangerous professions. Results of this recent study also found that regaining cognitive abilities that are diminished by shift work could take at least five years.

Source: J Marquié, Tucker P, Folkard S, Gentil C, Ansiau D. Chronic effects of shift work on cognition: findings from the VISAT longitudinal study. Occupational and Environmental Medicine. 2014.