In a study of sleep characteristics in 669 adults in Chicago who were compared by sex and race, investigators found that blacks got less sleep than whites, while men got less sleep than women.
Furthermore, the wealthier you are, the more sleep you're likely to get, Dr Diane S Lauderdale of the University of Chicago and her colleagues found.
"There was an expectation that people with very demanding jobs in terms of high status, high income, would be getting less sleep, and that was not true," Lauderdale told Reuters Health in an interview. The findings could help explain why blacks suffer from more health problems than whites, she added.
She and her colleagues monitored sleep in a group of men and women, most in their 40s, who were participating in a large study of heart disease risk. Fifty-eight percent were female and 44 percent were black.
Participants told the researchers how much sleep they thought they were getting, and then kept track of the time spent in bed and asleep using sleep logs.
Researchers also fitted them with wristwatch-like devices known as actigraphs that recorded their activity for three days, including two weeknights and one weekend night.
While people thought they were getting about seven hours of sleep nightly, they were really getting only about six hours, the researchers report in the American Journal of Epidemiology.
On average, white women slept 6.7 hours a night, white men slept 6.1 hours, black women slept 5.9 hours, and black men slept 5.1 hours nightly. The racial and sex differences remained even after the researchers factored in the effects of socioeconomic factors such as employment and lifestyle.
The amount of sleep people got increased with their income, and this effect was stronger for the black participants than the whites.
There are a number of potential explanations for the findings, Lauderdale noted. People who make less money may have more worries that prevent them from sleeping well. They could be living in noisier, less comfortable environments, and they may have more health problems.
The racial and economic sleep differences detected in this study could help explain the well-known disparities in health that exist between blacks and whites, she added.
"There are so many inequalities in health -- sleep can be involved in that," Lauderdale explained. "Sleep seems to be related to social differences in a way that we never realised."
SOURCE: American Journal of Epidemiology, July 1, 2006.