African American women who work night shifts are significantly more likely to develop diabetes than those who have never worked nights, possibly because of sleep cycle disruptions, says a new study by researchers from Boston University’s Schools of Public Health and Medicine.
The study, published in Diabetologia, the journal of the European Association for the Study of Diabetes, found that the risk of diabetes increased as women spent more years working night shifts. Relative to never having worked nights, the increased risk of developing diabetes was 17 percent for one to two years of night shift work, 23 percent for three to nine years, and 42 percent for 10 or more years.
After adjusting for BMI (body mass index) and lifestyle factors, such as diet and smoking status, the association between increasing years of night shift work and diabetes risk remained statistically significant, with a 23 percent increase in those who had worked night shifts for 10 years or more, compared to those who never worked the night shift.
“Even though lifestyle factors and BMI explained a major part of the association of shift work with incident diabetes, women with a long duration of shift work had an increased risk of diabetes after control for those factors, suggesting the presence of additional causal pathways,” the authors wrote. They noted that shift work is associated with disrupted circadian rhythms and reduced duration of sleep.
“Similar to the effects of jet lag, which are short term, shift workers experience fatigue, sleepiness during scheduled awake periods, and poor sleep during scheduled sleep periods. These alterations in the normal sleep–wake cycle have profound effects on metabolism,” they said. “Even after many years of night shift work, circadian rhythms do not fully adjust to the shifted sleep–wake cycle. The metabolic effects of long-term shift work likely underlie a part of the association with diabetes.”
Some previous studies have investigated the link between night shift work and diabetes—among them, the Nurses’ Health Study of mostly white women and another study in Sweden. However, body mass index accounted for most of the association in previous studies. With the increased prevalence of diabetes in black women in the US—12.6 percent, compared to 4.5 percent among whites—the authors decided that this potential association should be explored among black women.
The researchers followed more than 28,000 women enrolled in the long-running Black Women’s Health Study who were free of diabetes in 2005 and provided information about their work routines. The women were followed for incident diabetes during the next eight years. Thirty-seven percent reported having worked the night shift, with five percent having worked that shift for at least 10 years. During the eight years of follow-up, there were 1,786 incident diabetes cases.
The study found that when black women who reported ever working the night shift were compared to those who had never worked it, they had a 22 percent increased risk of developing diabetes. After adjusting for BMI and lifestyle factors, the increased risk was 12 percent.
The authors also found that the association was stronger in younger women than in older women. Working night shifts for 10 or more years relative to never working the night shift was associated with a 39 percent higher risk of diabetes among women younger than 50, compared with a 17 percent higher risk in women aged 50 years or older.
The study was led by Varsha Vimalananda, a researcher with the Center for Health Organization and Implementation Research, Edith Nourse Rogers Memorial Veterans Affairs Medical Center in Bedford, Massachusetts, and an assistant professor of medicine at BU School of Medicine.
“For most shift workers, simply quitting their jobs is not likely to be a feasible option for prevention,” Vimalananda said. “The burden of prevention may therefore lie at a social or employer level—for example, through avoidance of shift work when possible.” Vimalananda and her co-authors, most affiliated with BU’s Slone Epidemiology Center, wrote in the study that the exact mechanisms by which sleep disruption might impact diabetes risk are unclear. In animal models, circadian disruption in susceptible rats “led to more rapid loss of beta cell function and increased beta cell death, resulting in decreased beta cell mass, decreased glucose-stimulated insulin secretion and accelerated development of diabetes,” they wrote.
Noting a “high prevalence of shift work” among African Americans in the US—about 35 percent—they said the increased diabetes risk had important public health implications. “Because shift work may be unavoidable,” Vimalananda said, “continued research is needed that would help to facilitate circadian adaptation to shift work and other sleep disruptions.”
The other study authors included James L. Rosenzweig, associate professor of medicine; Julie R. Palmer, professor of epidemiology; Lauren A. Wise, associate professor of epidemiology; Lynn Rosenberg, professor of epidemiology; Edward A. Ruiz-Narváez, assistant professor of epidemiology; and Hanna Gerlovin, a PhD student in biostatistics. Palmer, Wise, Rosenberg, and Narváez are all affiliated with the Slone Epidemiology Center, which conducts the Black Women’s Health Study.
The study was supported by funding from the National Institute on Minority Health and Health Disparities and the National Cancer Institute.