No one likes to walk into work after just a few fitful hours of sleep. But now there's evidence that not getting enough sleep may have more serious consequences than dark circles under your eyes the next morning. Researchers at the University of Chicago report in the Journal of the American Medical Association that too little sleep can promote calcium buildup in the heart arteries, leading to the plaques that can then break apart and cause heart attacks and strokes.
The University of Chicago team documented for the first time exactly how much of a risk shortened shut-eye can be one hour less on average each night can increase coronary calcium by 16%. Among a group of 495 men and women aged 35 to 47, 27% of those getting less than five hours of sleep each night showed plaque in their heart vessels, while 11% of those sleeping the recommended five to seven hours did, and only 6% of subjects sleeping more than seven hours each night showed such atherosclerosis. "We were surprised by the findings," says Diane Lauderdale, a professor of health studies at the University of Chicago and lead author of the study. "We really were not expecting to find an association at all, and certainly not one that was this strong." (See the Top 10 Medical Breakthroughs of 2008.)
Lauderdale and her team had reason to be skeptical. While the connection between sleep and heart disease is of growing interest to researchers, earlier studies had been inconclusive, and plagued by biases, including the fact that most of the trials relied on people's self-reported accounts of their sleeping habits. The scientists knew that teasing apart the myriad processes that contribute to sleep, and then drawing scientifically sound connections between them and the host of things that can trigger heart disease, would be difficult at best. So the Chicago team isolated the most common confounding variables that could explain both poor sleep and heart problems, such as smoking, alcohol, and other medical conditions, and also found a way to record, as accurately as possible, the amount of sleep that the subjects got each night. Each volunteer wore a wrist monitor that measured and recorded activity at 30 second intervals; when the monitor was quiet, the subject was asleep.
While Lauderdale acknowledges that her results are far from the last word on sleep and heart disease, the study does suggest that doctors and patients should consider sleep in addition to the more familiar hazards for the heart such as high cholesterol, hypertension and diabetes. In Lauderdale's analysis, one additional hour of sleep was equivalent to lowering systolic blood pressure by 16.5mm Hg. "We have enough evidence from this study and others to show that it is important to include sleep in any discussion of heart disease," says Dr. Tracy Stevens, spokesperson for the American Heart Association and a cardiologist at Saint Luke's Mid-America Heart Institute. "We talk about the traditional risk factors, and now the other important thing we need to include is sleep." (See pictures of how animals sleep.)
Exactly how a lack of sleep is feeding plaque in the heart arteries isn't yet clear, but one explanation may involve inflammation. Too little sleep can raise cortisol levels, which fuels inflammation that can destabilize plaques. Once these deposits rupture, they can block vessels in the heart or brain, causing a heart attack or stroke. While the Chicago team did not track levels of cortisol to test this theory, future studies might.
A simpler explanation might involve blood pressure. In general, blood pressure dips during sleep, and over a 24 hour period, people sleeping less will have shorter periods of lower blood pressure, thus increasing their tendency to dislodge any unstable plaques.
Whatever the reason might be, the results of this study make it clear that sleep isn't just for dreamers. Getting enough sleep might just save your heart.