They found that sleep duration and sleep efficiency were "remarkably lower" than values reported in most previous studies, noted Stuart F. Quan of the University of Arizona in a commentary.
The researchers were particularly surprised by the short span and poor quality of sleep among African-American men -- 5.1 hours a night and 73 percent sleep efficiency.
"Although sleep scientists have generally accepted that the average sleep duration of Americans has been declining in parallel with our transformation to a frenetic 24-hour society," Quan wrote, "most sleep clinicians would consider those values indicative of sleep deprivation even by current standards."
Lack of sleep has long been connected with reduced ability to concentrate, trouble learning, decreased attention to detail and increased risk of motor vehicle accidents. More recent studies have tied chronic partial sleep deprivation to medical problems, including obesity, diabetes and hypertension.
This study may someday connect sleep loss to coronary artery disease. The 669 volunteers, aged 38 to 50, were recruited from the Chicago site (based at Northwestern University) of the CARDIA study, an ongoing project, begun in 1985, designed to assess long-term cardiovascular risk factors.
Although the study found significant variation based on race, sex and income it was not designed to get at the causes of those differences.
"People who make more money may have fewer worries," Lauderdale suggested, "or they may have more control over their sleep environment."
The findings, however, are "consistent with sleep being on the causal pathway between socioeconomic status (or race) and disease risk," the authors conclude.