Women who work rotating night shifts may have a higher risk of developing Type 2 diabetes, a large new study suggests.

Earlier studies found a link between shift work and risk of cancer, cardiovascular disease, and obesity among shift workers but few large studies looked at have look at female shift workers' risk of Type 2 diabetes.

Rotating night shift is becoming more common in industrialized countries, researchers said. The longer that nurses worked rotating night shifts, the higher their risk of developing Type 2 diabetes, researchers found. Rachel Denny Clow/Corpus Christi Caller-Times/AP

In Tuesday's issue of the journal PLoS Medicine, published online by the Public Library of Science, An Pan of the Harvard School of Public Health his co-authors analyzed health and lifestyle data on more than 69,000 women aged 42 to 67 and more than 107,000 women aged 25 to 42.

The longer that participants in the Nurses Health Study worked rotating night shifts — defined as at least three nights per month plus other days and evenings in that month — the greater their risk was of developing Type 2 diabetes.

During the 18 to 20 years of follow-up, 6, 165 cases of Type 2 diabetes were found in the older group and 3,961 were found among the younger women.

The risk of developing Type 2 diabetes were:

• 0.99 times higher for working one to two years of rotating shifts.
• 1.17 times higher for those who worked three to nine years of rotating shifts.
• 1.42 times higher among those working 10 to 19 years of rotating shifts.
• 1.64 times higher for those who worked 20 or more years on rotating shifts.

"Our current analysis provides compelling evidence that an extended period of rotating night shift work is associated with a moderately increased risk of Type 2 diabetes, which was not completely explained by body mass index," the study's authors concluded.

Diet, sleep patterns

The findings emphasize the importance of improving diet and lifestyle to prevent Type 2 diabetes among this high-risk group, said study author Frank Hu of Harvard Medical School in Boston.

The researchers suggested several possible ways that rotating shift work might influence diabetes risk.

For example, rotating night shift work is associated with chronic alignment problems for the body's circadian rhythms, which help regulate sleep-wake cycles, energy metabolism and hormone secretion, the team said.

The long-term effects of eating habits in rotating shift workers is unknown, they said. Other factors such as stress from working atypical work hours are also possibilities.

Two smaller studies of male Japanese workers also suggested that shift work is tied to diabetes risk, but it may be difficult to apply the latest findings to men and other racial or ethnic groups besides the study population of mainly white female nurses, the authors cautioned.

A journal editorial accompanying the study called the findings of "potential public health significance," adding that preventative strategies for rotating night shift workers should be considered.

Prof. Joan Tranmer of Queen's University in Kingston, Ont. has studied how female nurses also face higher risks of cardiovascular disease.

Maintaining a healthy lifestyle through physical activity, weight and diet seems to help protect against harmful influences such as shift work, said Tranmer, a former nurse who did shift work for 30 years.

But not all women who work shifts become unhealthy, she noted.

"It could be that chronic shift work is associated with prolonged circadian disruption, or sleep disruption, or job stress, or poor lifestyle behaviours, all pathways which need to be explored in order to plan effective prevention and treatment strategies," Tranmer said in an email.

As she waits tables at a restaurant in Toronto, Grace Mangos said she can relate to the lifestyle findings.

"You're eating at all hours of the night," Mangos recalled of her night shifts at the eatery that serves breakfast 24 hours a day. Mangos said she gained 20 pounds during those four years.

"When you're sleep deprived, you don't tend to reach for broccoli, you're tending to reach for pancakes."

Funding for the study was provided by the U.S. National Institutes of Health and the U.S. National Heart, Lung, and Blood Institute.

With files from CBC's Lorenda Reddekopp