Black women working a night shift face a significantly higher risk of diabetes, and the risk gets greater the more years that work pattern endures, a recent study indicates.

That risk increased 17 percent after one or two years of night work compared to black women who never worked a night shift, reached 23 percent after 3 to 9 years, and was at 42 percent after 10 or more years, researchers at the Slone Epidemiology Center at Boston University reported in their study.

"Our findings from the Black Women's Health Study contribute to the observational literature that consistently demonstrates a relationship between disruption of circadian rhythms and long-lasting adverse effects on metabolism, such as obesity and type 2 diabetes mellitus," says study lead author Dr. Varsha G. Vimalananda.

The results come from a study of more than 28,000 black women, ages 21 to 69, who were free of diabetes in 2005 and who were tracked for 8 years from that date.

In that follow-up period, almost 1,800 cases of diabetes were diagnosed among them.

Of those 28,000, 37 percent reported at least some periods of night shift work, and 5 percent said they had done night shift work for at least 10 years.

While the mechanism behind an increased diabetes risk is not yet clear, the researchers point out that shift work is linked with disruption of circadian rhythms and reduction in sleep duration, both of which can affect metabolism.

Previous investigations have shown disruption in circadian rhythm is associated with increased insulin resistance, inflammation and increased glucose and insulin levels, the researcher point out.

The researchers said the study was prompted in part by the knowledge that in the United States diabetes is twice as prevalent in black women, at 12.2 percent, than in white women (4.5 percent.)

Thirty-five percent of non-Hispanic black Americans are part of the 8 million Americans doing shift work, defined for the study as work occurring outside of normal daytime working hours.

The discovery of a higher risk of diabetes even after adjusting for factors such as lifestyle and weight status is significant evidence that disruption of circadian rhythm and sleep deprivation may play a role, the researchers say.

It's something both doctors and patients should be aware of, Vimalananda says.

"Clinicians should discuss the adverse metabolic consequences of shift work and sleep disruption and sleep loss in general with patients, [who] may have some choices in their work schedules and may be able to make behavioral modifications at home that promote sufficient sleep at night," he says.