An extra hour of sleep at night makes most people get up next morning on the right side of the bed. And according to a study published in today’s JAMA, people who habitually get that extra hour or more may have better cardiovascular health.

Longer sleep, according to research led by University of Chicago epidemiologist Dr. Diane Lauderdale, PhD, makes for substantial reduction in the risk of developing hardened, or calcified, arteries. Coronary artery calcification involves the accumulation of calcified plaques in artery walls. The new research has found an unexpected link between this condition and sleep.

“We have found a robust and novel association between objectively measured sleep duration, ” Lauderdale writes, “and 5-year incidence of coronary artery calcification. One hour more of sleep decreased the estimated odds of calcification by 33%.”

The effects of sleep were measured on 450 individuals with follow-up for 5 years. The benefit of reduced calcification of the arteries turned out to be greater from extra sleep than from any other factor studied. “The modeled effect of 1 additional hour of sleep on the odds of incident calcification,” LAuderdale writes,”was equal to the modeled effect of a 16.5 mm Hg decrease in systolic blood pressure.”

The study was not perfect, Lauderdale emphasizes, but everything was done to make results as accurate and as close to real life conditions as possible. The researchers used a relatively non-invasive method of monitoring human rest/activity cycles, called actigraphy, in this case by means of a wristband sensor. Wristband actigraphy is much more accurate than self-reported sleep and in this group, results remained consistent from year to year. At the same time:

actigraphy is unable to measure potentially important dimensions of sleep such as sleep stages, which may underlie the apparent association between duration and calcification. Sleep quality is multidimensional, and there is no perfect metric for measuring it; the apnea-hypopnea index from polysomnography is probably the closest to a criterion standard. Actigraphy-measured fragmentation has not been widely used in research, although 1 recent study did find a significant correlation between it and obesity in the elderly. How actigraphic fragmentation relates to other measures of sleep quality remains unclear.

Need for a closer look at the independent effects of sleep apnea is one of this study’s admitted limitations. Also, the researchers say, “too few participants had calcification at baseline for us to examine the rate of further calcification among them” and “our first wave of sleep measures was taken more than halfway through the period between baseline and follow-up. While calcification may have occurred before sleep measurements, there is no obvious reverse causation mechanism. Further, the level of calcification was not known by the participants at the time of the sleep assessment.”

A further possible limitation of this study lies in the fact that while hardening of the arteries or calcification tends to increase over time, in its early stages it does not amount to a serious health risk and “coronary events may not necessarily follow.” Nonetheless, calcification is “a potent risk factor for coronary events.”

More research is needed, Lauderdale implies, to correlate the effects of sleep duration with socioeconomic status and with levels of inflammatory markers (fibrinogen and interleukin 6), glucose, and the stress hormone cortisol. Also it is not yet clear whether sleeping more than 8 hours becomes counterproductive.

Overall, this study found good evidence to suggest that allowing time for 8 hours sleep is important for health.

Source:

Short Sleep Duration and Incident Coronary Artery Calcification
Christopher Ryan King, BS; Kristen L. Knutson, PhD; Paul J. Rathouz, PhD; Steve Sidney, MD, MPH; Kiang Liu, PhD; Diane S. Lauderdale, PhD. JAMA. 2008;300(24):2859-2866.

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CREDITS: Photo of father and son sleeping by Marta Dehnel, Poland.
Photo of man asleep on ferry by Derek Jones, UK.