Compared to whites, African Americans are more likely to suffer from a constellation of health problems referred to as “cardiovascular and metabolic diseases:” high blood pressure, diabetes, and stroke. Things like diet, exercise, and smoking contribute to those conditions, but when researchers control for those behaviors, the disparity persists. Now scientists are examining an unexpected factor that could be driving these disparities in heart disease: sleep.

Think of sleep as a time when the body tidies up its hormonal systems. People who consistently don’t get enough sleep have increases in ghrelin, a hunger hormone, and decreases in leptin, a hormone that helps people feel sated. That might lead to increased eating during the day. Even if it doesn’t, sleep deprivation wreaks havoc on other key hormones and proteins, like insulin and the inflammatory markers C-reactive protein and interleukin-6—which are, in turn, linked to cardiovascular and metabolic diseases.

David Curtis, a human development researcher at Auburn University, decided to try to determine whether differences in sleep could explain some of these racial disparities in cardiometabolic diseases. He and researchers from Northwestern University and the University of Wisconsin recruited 426 white and African American men and women and equipped them with Fitbit-type devices that can monitor sleep. They took some biological measurements, such as blood pressure, waist circumference, and insulin resistance, and then measured how long they slept each night for seven nights. It wasn’t enough just to be in bed: The wrist-bands they wore also measured how often the participants woke up in the night.

In the resulting study, published this week in the Proceedings of the National Academy of Sciences, they found that African-Americans got about 40 minutes less sleep each night than the white participants did, and their sleep was about 10 percent less “efficient”—meaning they were more likely to wake up in the middle of the night or to have a hard time falling asleep. They also had higher biological risk factors for cardiovascular disease—things like higher blood pressure and a larger waist circumference. But the most surprising thing was how those two were related: According to their model, about half of the racial difference in cardiometabolic disease risk could be explained by the sleep deprivation among African Americans—and really, only explained by sleep deprivation among black women, not men. When they looked at the data by sex, the relationship was only significant for the women in the sample.

“The study and its conclusions are in line with previously published data showing similar patterns that African Americans have shorter habitual sleep duration and poorer habitual sleep quality,” said Garth Graham, the president of the Aetna Foundation and former director of minority health at the Department of Health and Human Services. A meta-analysis of 14 studies found that African-Americans sleep worse and for shorter periods of time than white people do.

Of course, cardiovascular disease can also cause sleep disorders, like sleep apnea, that can make people sleep fitfully. To control for that, Curtis and his co-authors excluded people with sleep disorders and those who already had heart disease and diabetes. Even in these, relatively healthy subjects, sleep deprivation still explained cardiometabolic disease risk.

So what’s causing these racial differences in sleep time and quality? This study doesn’t say, but based on past research, Curtis speculates that it could be neighborhood crime or economic stress. People don’t feel safe in their neighborhoods, or they feel too worried about money to sleep. In past studies, Curtis and his colleagues have found that people in disadvantaged neighborhoods are more likely to wake up in the night, and in many major cities black families are more likely than white families to live in concentrated poverty. Perceived racial discrimination is also associated with sleep disturbances.

It’s unclear why the relationship between sleep and health risks was stronger for women than for men. Pregnancy could have something to do with it. Or, it could be that “being a black female in the United States is inherently more stressful than being just a female or being just black,” Curtis said. “There are multiple identities that are disadvantaged in society, and that can lead to health risks.”

Curtis sees his findings as pointing to an important sleep gap between black and white Americans. Knowing it exists can help build the case for public-health interventions on sleep, just like those already in existence for food and exercise. For example, interventions like tai chi or cognitive behavioral therapy might help people sleep better in the short term, even if it doesn’t resolve the root cause of their worries.

“If our society is committed to reducing racial health disparities,” Curtis said, “our national health promotion efforts need to consider sleep seriously.”