By: Pam Harrison
February 22, 2018

Shift work, especially irregular shifts with a lot of nights, increases the risk of type 2 diabetes, and the more nights worked, the greater the risk, new research shows.

"Shift work, particularly night shifts, disrupts social and biological rhythms, as well as sleep, and has been suggested to increase the risk of metabolic disorders, including type 2 diabetes," says lead author Celine Vetter, PhD, director, Circadian and Sleep Epidemiology laboratory at the University of Colorado, Boulder.

"Our study is one of the first to show a dose–response relationship, where the more often people work nights, the greater their likelihood of having the disease," she noted in a press release from her institution.

However, of interest, she and her colleagues also found that those who worked permanent night shifts did not have a significantly increased risk for diabetes.

The research was published online February 12 in Diabetes Care.

How Do Different Work Schedules Impact Risk of Diabetes?

Vetter and colleagues note that about 15 million Americans work permanent night shifts, rotating shifts, or shifts with irregular schedules. Recent studies have found associations between such shift work and cardiovascular disease, diabetes, and cancer, but few have looked specifically at how different work schedule characteristics impact risk.

For this analysis, the research team analyzed data on 272 217 participants in the UK Biobank database, 77 000 of whom provided a lifetime history of employment.

Compared with people who worked only during the day, all shift workers were more likely to have type 2 diabetes. Adjustment for many confounding variables including body mass index attenuated the risk but it remained significant, the study authors point out.

Compared with day workers, for those who worked no nights or who had rare night shifts, the risk of type 2 diabetes was 15% higher; for those who worked 'some' nights, the risk was 18% higher; and for those who usually worked nights, the risk was 44% higher.

The risk of type 2 diabetes increased as the number of night shifts per month increased, although the risk was not completely linear.

Night Shifts Worked Per Month and Type 2 Diabetes Risk

Night Shifts Worked Per Month vs No Night ShiftsType 2 Diabetes Risk

< 3 nights/month24% higher risk

3–8 nights/month11% higher risk

> 8 nights/month36% higher risk

Physical Activity Plays Into the Mix Too

Perhaps not unexpectedly, levels of physical activity also affected a worker's risk of developing type 2 diabetes. For example, the risk of type 2 diabetes was "consistently higher" among workers who reported low levels of physical activity and who worked any night shift.

In contrast, the risk of type 2 diabetes was only higher when workers with higher levels of physical activity reported work schedules that usually included night shifts.

Interestingly as well, the risk of type 2 diabetes was only elevated among workers who had been exposed to shift work for less than 10 years, but not among those who had been exposed to longer durations of shift work.

Investigators speculate that workers who developed diabetes might be more prone to quit shift work because they are sicker than those without diabetes.

Genetic Risk Doesn't Modify Association

The team also developed a genetic risk score for a subset of study participants and analyzed the data to see if exposure to shift work modified an individual's genetic risk for type 2 diabetes.

"As expected, a higher genetic risk for diabetes was associated with a higher type 2 diabetes likelihood," they observe. But the association between genetic type 2 diabetes predisposition and type 2 diabetes odds was not modified by shift work exposure. This is "a novel finding that warrants replication," say the researchers.

Vetter noted that people often have no choice over whether they work nights or not.

But she suggested that workers who are subject to rotating night shifts maintain a healthy weight and get enough sleep and exercise to offset the health risks of having to work at least some nights.

Approximately 10% of the workforce work some nights including permanent night shifts, rotating shifts, and irregular schedules.

"Our study findings represent another puzzle piece in this quest towards healthier work schedule design," the researchers conclude.

The study was funded by the National Institutes of Health and the University of Manchester. Vetter has reported no relevant financial relationships. Disclosures for the other authors are listed in the article.

Diabetes Care. Published online February 12, 2018.

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