Study reveals why night shift workers face much higher risk of cancer

SPOKANE, Wash. — Working overnight has been linked to various health conditions, including irregular heartbeat and even cancer. In a recent study, researchers have pinpointed likely reason for the increase in cancer risk of people who regularly work night shifts.

The study by scientists at Washington State University’s Health Sciences sleep laboratory shows that the normal 24-hour rhythm is disrupted in those who regularly worked at night. This causes abnormal activity of certain genes associated with cancer, increasing the risk of DNA damage.

Researchers also say a reduction in the mechanisms used to repair DNA limits the body’s ability to combat any damage.

“There has been mounting evidence that cancer is more prevalent in night shift workers, which led the World Health Organization’s International Agency for Research on Cancer to classify night shift work as a probable carcinogenic. However, it has been unclear why night shift work elevates cancer risk, which our study sought to address,” explains co-corresponding author Shobhan Gaddameedhi, an associate professor with North Carolina State University’s Biological Sciences Department and Center for Human Health and the Environment, in a statement.

A night shift schedule is associated with increased DNA damage and misalignment of the DNA repair mechanism, providing a possible explanation for the elevated risk of cancer in night shift workers. (Credit: Bala Koritala)

Gaddameedhi and researchers at the WSU Sleep and Performance Research Center collaborated with the Pacific Northwest National Lab (PNNL) to determine the method by which the body’s biological clock alters gene activity.  Similar to the “clock” that controls brain activity, there is an integral biological clock in almost every cell of the body. This regulates genes within each cell called “clock genes,” which are expressed differently according to the time of day.

The team speculates that cancer genes may also be expressed in a similar way, becoming more or less active when the cycle is disrupted. 

Searching for the link to cancer

For the study, 14 healthy volunteers simulated day and night shifts for a week at the sleep laboratory. Seven volunteers were put on a night shift routine for 3 days, while the rest simulated a normal day shift routine. To observe the internal biological rhythms produced by the shift work, all volunteers had to stay awake for the next 24 hours, during which they were partially reclined in chairs while the amount of light remained the same.
To ensure the study wasn’t dependent on other factors, all volunteers ate the same foods each hour, and the room was kept at a constant temperature. Researchers took samples of blood every three hours.

The blood samples were analyzed for white blood cells which showed a difference in the activity of the genes associated with cancer. Also, researchers noted a disparity in the cycle of the genes associated with DNA repair. These genes were upregulated during the day shift routine, however, in those simulating night shifts, these genes were downregulated and seemed to lose their rhythm.

These results pivoted the focus of the study to possible effects caused by varying expression of the genes related to cancer. Researchers found an increase in DNA damage of those that worked during the night. Moreover, radiation was used to test the susceptibility of white blood cells taken from each volunteer, which showed more damage to the DNA of night shift workers. Previous studies have found cell vulnerability to radiation to be a marker for the increased risk of cancer.

“Taken together, these findings suggest that night shift schedules throw off the timing of expression of cancer-related genes in a way that reduces the effectiveness of the body’s DNA repair processes when they are most needed,” says co-corresponding author Jason McDermott, a computational scientist with PNNL’s Biological Sciences Division.

‘Night shift workers face considerable health disparities’

The next step is to analyze the DNA of those who have worked real shift work for years. The team would like to find out whether damaged DNA remains in the cells over an extended period of time. This would indicate a greater risk of cancer developing. 

If the present findings are confirmed in actual shift workers, this research might potentially be utilized to create preventive methods and medicines to promote DNA repair. It might also serve as the foundation for efforts to better time cancer treatment so that it is delivered when it is most effective and has the fewest adverse effects, a process known as chronotherapy that would require strategic adjustment to coincide with night workers’ internal clocks.

“Night shift workers face considerable health disparities, ranging from increased risks of metabolic and cardiovascular disease to mental health disorders and cancer,” concludes co-senior author Hans Van Dongen, director of the WSU Sleep and Performance Research Center. “It is high time that we find diagnosis and treatment solutions for this underserved group of essential workers so that the medical community can address their unique health challenges.”

This study is published online in the Journal of Pineal Research.

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