A recent study has uncovered another possible risk factor for the development of type 2 diabetes: working long hours in low-paying jobs.

In a study published this week in the Lancet Diabetes & Endocrinology, researchers found that people who work more than 55 hours per week performing manual work or other low socioeconomic status jobs face a 30 percent greater risk of developing type 2 diabetes when compared to those working between 35 and 40 hours per week. The association remained even after researchers accounted for risk factors such as smoking, physical activity levels, age, sex and obesity as well as after they excluded shift work, which has already been shown to increase type 2 diabetes risk. The study is the largest so far to examine the link between long working hours and type 2 diabetes.

To conduct the study, researchers examined data from 23 studies involving more than 222,000 men and women in the U.S., Europe, Japan and Australia who were followed for an average of more than seven years. While on the surface, researchers found a similar type 2 diabetes risk among those who worked more than 55 hours per week and those working a more standard 35-40 hour week, more in-depth analysis revealed that workers in low socioeconomic jobs did, indeed, face a significantly higher risk. In other words, the association between long work hours and higher type 2 diabetes risk was only apparent among low-income groups. In a related commentary published in the same journal issue, authors Orfeu Buxton, a lecturer at Harvard Medical School, and Cassandra Okechukwu, an assistant professor with the Harvard School of Public Health, write:

(Study lead author Mika) Kivimaki and colleagues’ finding of an association only in the low socioeconomic status group suggests a possible role of sleep as a mediator; both inadequate sleep quality and quantity are strongly predictive of incident type 2 diabetes. A recent study showed that the association between short sleep duration and diabetes can be partly attenuated by inclusion of socioeconomic status. Working more consistent and less disruptive hours might explain the lack of associations for the individuals in the high socioeconomic status group. A related idea is the role of social jet lag — long working hours and short sleep during the weekdays combined with delayed sleep times on the weekend — that disrupts the circadian timing system and adversely affects metabolism. Laboratory studies have identified mechanisms by which disruption of the circadian system increases the risk of diabetes. Night work, a cause of circadian disruption, is also associated with higher risks of type 2 diabetes.

The study’s authors discussed three possible ways in which working longer hours could manifest into such a significant health risk for low-income workers. The first is that long work hours among low-income workers may be a marker of personal hardships and reflect other confounding variables, while workers with a high socioeconomic status do not experience such hardships and so working longer hours is not hazardous. The second hypothesis is that working longer hours may leave low-income workers with little time to engage in health-protective behaviors, such as getting enough sleep, exercising and socializing. The third possibility is that working longer hours is simply dangerous to a person’s health, but the authors write that “the fact that we do not see (the same health risks among workers with high socioeconomic status) suggests that the association is driven by confounding or indirect effects on other risk factors.”

Still, the study raises the question of whether work conditions should be considered in efforts to prevent diabetes. Globally, more than 285 million people have type 2 diabetes and that burden is expected to grow to 439 million by 2030. In the U.S. alone, more than 25 million people have diabetes and if trends continue, one in three U.S. adults will be living with the costly chronic disease by 2050.

To read the full study, visit the Lancet.

Kim Krisberg is a freelance public health writer living in Austin, Texas, and has been writing about public health for more than a decade.