Participants in a study who slept on average an hour longer per night than other participants had an associated lower incidence of coronary artery calcification, which is thought to be a predictor of future heart disease, according to a study in the December 24/31 issue of JAMA.
Risk factors for coronary artery calcification (the accumulation of calcified plaques visible by computed tomography [a method of imaging body organs]) include established heart disease risk factors such as male sex, older age, glucose intolerance, tobacco use, dyslipidemia (disorders of lipoprotein metabolism, which includes high cholesterol levels), high blood pressure, obesity, raised inflammatory markers and attaining a low educational level. Recent data suggest that sleep quantity and quality are connected to several of these risk factors. "However, some of these correlations have only been documented in studies in which sleep is measured by self-report, which may be biased or insufficiently accurate," the authors write.
Christopher Ryan King, B.S., of the University of Chicago, and colleagues tested whether objectively measured sleep duration predicted the development of calcification over 5 years of follow-up. The study included 495 participants from the Coronary Artery Risk Development in Young Adults [CARDIA] study, who were black and white men and women age 35-47 years. At the start of the study in 2000-2001, the participants had no evidence of detectable coronary calcification on computed tomography scans.
Potential confounders (factors that can possibly influence the results; age, sex, race, education, apnea risk, smoking status) and mediators (lipids, blood pressure, body mass index, diabetes, inflammatory markers, alcohol consumption, depression, hostility, self-reported medical conditions) were measured at both the start of the study and at 5-year follow-up. Sleep metrics (wrist actigraphy [monitors rest and activity] measured sleep duration and sleep fragmentation, daytime sleepiness, overall sleep quality, self-reported sleep duration) were examined for association with new calcification, based on computed tomography performed in 2005-2006.
The incidence of calcification at 5 years was 12.3 percent (n = 61 participants). After adjusting for age, sex, race, education, smoking and apnea risk, the researchers found that one hour more of sleep per night decreased the estimated odds of calcification by 33 percent. The magnitude of the observed association was similar to sizable differences in established coronary risk factors (e.g., 1 additional hour of sleep reduced risk similarly to a reduction of 16.5 mm Hg in systolic blood pressure). No potential mediators appreciably altered the magnitude or significance of sleep. Alternative sleep metrics were not significantly associated with calcification.
"We have found a robust and novel association between objectively measured sleep duration and 5-year incidence of coronary artery calcification," the authors write. "This study further demonstrates the utility of a simple objective measure of sleep that can be used at home. Future studies will be needed for crucial extensions to these results. First, these results need confirmation in other cohorts. Second, does sleep moderate the rate at which calcification accumulates? Third, will objective sleep tie to coronary disease event outcomes over the long term? While calcification predicts such outcomes, it is difficult to know how and if the predictors of calcification themselves will determine outcomes, or if their impact will be purely mediated by their effect on calcification. Finally, if this association is born out, interventional studies will be needed to guide clinical advice."
(JAMA. 2008;300:2859-2866. Available pre-embargo to the media at www.jamamedia.org)
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