According to a study published in the December 24/31 issue of JAMA, sleeping an hour longer each night may lead to a lower incidence of coronary artery calcification - a predictor of future heart disease.

Coronary artery calcification is characterized by an accumulation of calcium plaques that detected computed tomography scans of the heart. As a risk factor for atherosclerotic heart disease, coronary artery calcification also shares many of the same risk factors for heart disease including:
Being male Being older Glucose intolerance Tobacco use High cholesterol levels and other disorders of lipoprotein metabolism (dyslipidemia) Hypertension (high blood pressure) Obesity Raised inflammatory markers A low educational level In some recent studies, sleep quantity and quality has been found to be related 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," write the authors of the study.

To test if an objectively-measured sleep duration was linked to the development of calcification over a 5 year period, Christopher Ryan King, B.S. (University of Chicago) and colleagues conducted a study with 495 individuals. The participants were part of the Coronary Artery Risk Development in Young Adults [CARDIA] study, a sample of black and white men and women between the ages of 35 and 47. The study began in 2000-2001, a time in which none of the participants demonstrated evidence of detectable coronary calcification on computed tomography scans.

Baseline data and follow-up data were collected on factors that could potentially make it more difficult for the researchers to tease out the effect of sleep. These possible confounders included age, sex, race, education, apnea risk, smoking status, lipids, blood pressure, body mass index, diabetes, inflammatory markers, alcohol consumption, depression, hostility, and other self-reported medical conditions. Measurements of sleep duration, sleep fragmentation, daytime sleepiness and overall sleep quality were provided by wrist actigraphy (a tool that monitors rest and activity) and self-reports. The researchers focused the study on the effect that these sleep metrics had on new calcification as detected by computed tomography scans in 2005-2006.

Results indicated that 12.3% (61 participants) developed calcification after 5 years. After controlling for confounding factors like age, sex, race, education, smoking and apnea risk, the researchers report that one additional hour of sleep per night lowered the estimated odds of calcification by 33%. In addition, the magnitude or significance of sleep was not appreciably altered by potential mediators. Sleep metrics other than duration did not appear to be significantly linked to the risk of calcification.

"We have found a robust and novel association between objectively measured sleep duration and 5-year incidence of coronary artery calcification," write King and colleagues. "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."

Short Sleep Duration and Incident Coronary Artery Calcification
Christopher Ryan King; Kristen L. Knutson; Paul J. Rathouz; Steve Sidney; Kiang Liu; Diane S. Lauderdale
JAMA (2008). 300[24]: pp. 2859-2866.
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: Peter M Crosta