In a previous study, Lauderdale and colleagues used actigraphy and nightly logs to study, on average, how long people spent in bed (7.5 hours), how long it took them to fall asleep (22 minutes), how long they slept (6.1 hours), and their total sleep efficiency--time asleep divided by time trying to sleep in bed (81 percent).

Previous studies have correlated decreased sleep times with established risk factors for calcification, including high blood pressure, excess weight, and poor glucose regulation. But in this study, "after adjusting for age, sex, race, education, smoking, and apnea risk," the authors note, "longer measured sleep duration was associated with reduced calcification incidence."

The authors suggest three possible ways that shorter sleep could connect to calcification. First, there may be some factor not yet identified that can both reduce sleep duration and increase calcification. Second, although blood pressure measured during examinations did not seem to explain the association, blood pressure generally declines during sleep, so the 24-hour average blood pressure of those who sleep less may be higher, and that could lead to calcification, researchers found.

Finally, stress or a stress hormone like cortisol, which has been tied to decreased sleep and increased calcification, may play a role. Cortisol data were not available for all study participants.

"This was a small study and a new finding, so we would love to see it duplicated in another study population," Lauderdale said. "But there is enough here to make a point. Although there are constant temptations to sleep less, there is a growing body of evidence that short sleep may have subtle health consequences. Although this single study does not prove that short sleep leads to coronary artery disease, it is safe to recommend at least six hours of sleep a night," Lauderdale said.

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