One obvious remaining culprit, says Dr. Carnethon, is socioeconomics.

Because Chicago is still a fairly segregated city, “the blacks and Hispanics in our study were generally living in neighborhoods that are closer to freeways, so you have freeway noise, there’s more business noise at night, and there’s potentially more crime, which is stressful to people,” Dr. Carnethon said. People in lower-income neighborhoods are also more likely to have multiple jobs or to work odd hours, which can interfere with sleep.

The idea that differences in work and living conditions can explain the racial sleep disparities is a popular one among sleep experts. But studies that have accounted for those factors suggest a more complex reality.

One such study from 2005 — also taking place in Chicago — measured sleep among 669 participants while adjusting for education, income and employment status. In the end, black men on average still slept 82 minutes less per night than white women, who were found to sleep the best of anyone in the study.

Of course, isolating the real-life effects of social inequality can be tricky. “There are more subtle differences” among people than income and education, said Dr. Kristen Knutson, assistant professor of medicine at the University of Chicago and an author of the study. “We had no way to control for stress, and there are social stresses an African-American man might feel that a white man with the same income and education level wouldn’t.”

Mr. Stewart, the Brooklyn resident, said he did see discrimination as playing a role in his sleep problems.

“As a black person in America, even if you succeed in terms of education, you still have to deal with the inherent inequality of society,” said Mr. Stewart, an administrator for a program that exposes students in racial minorities to careers in science and math. “I don’t blame it on the majority — that’s just simplistic. But in general it’s not a fair thing, and you stress because of that.”

Sleep experts refer to this as the “autonomy” problem, and studies have shown it has an effect on sleep. “People who feel they have control over their lives were able to feel secure at night, go to sleep, sleep well, and wake up well in the morning and do it all over again,” said Dr. Lauren Hale, associate professor of preventive medicine at Stony Brook University on Long Island, referring to a study she conducted in 2009. “That’s part of the cycle not just for blacks and minorities, but other disadvantaged populations.”