The human heart requires a certain amount of sleep every night to stay healthy, and that link between sleep and heart health is stronger than researchers suspected, according to a report in the Journal of the American Medical Association.
Earlier studies had suggested a weak association between the amount of sleep people typically get and their risk of heart disease. But those studies were based on people's own reports of their sleep times, and self-reports of sleep are notorious unreliable. Researchers say people often overstate the amount of sleep they get.
University of Chicago scientists decided to look more carefully. They outfitted 500 volunteers in their early 40s with an $850 device called an actigraph. Worn on the wrist, the device measured participants' movements day and night.
Diane Lauderdale, the study's senior author, says when people are asleep the device registers a characteristic pattern of movement.
"It's not actually no movement," she says. "There is slight motion during sleep. It doesn't look the same as if you just take the watch off and put it on the counter."
The actigraphs gave Lauderdale's group an objective read-out of how much people actually sleep. The average for the entire group was about six hours. They spent seven hours in bed, on average, but it took time to fall asleep.
To measure people's underlying heart health, the researchers did computerized tomography, or CT, scans of their coronary arteries. The scans can reveal telltale deposits called calcifications, which are considered a strong marker of heart disease.
When they put it all together, the researchers got a surprising result. Among these healthy, middle-aged volunteers, those who averaged five or fewer hours of sleep had a much bigger incidence of silent heart disease.
"Twenty-seven percent of them developed coronary artery calcification over the five years of follow-up," Lauderdale says. "Whereas among the persons who slept seven hours or more, on average, only 6 percent developed coronary artery calcification."
In other words, the sleep-deprived people had 4.5 times the risk of heart disease — and that's after researchers subtracted out the effects of other known coronary risk factors, such as high cholesterol, high blood pressure, diabetes and smoking.
It remains to be seen why too-little sleep is linked to clogged coronaries. Maybe it has something to do with stress hormones. Lauderdale says other studies have shown that depriving people of sleep raises their levels of cortisol, one stress hormone.
Or maybe it's related to blood pressure.
"It's possible that people who are sleeping longer hours have a deeper or more sustained blood pressure dip during the night," Lauderdale says.
Dr. Sanjay Patel of Case Western Reserve Medical School, a sleep researcher not involved in the new study, thinks sleep might affect hormones such as leptin that regulate body weight, appetite and metabolism.
"Several studies have shown that if you sleep-deprive an individual for a few days, you have hormonal changes that tend to increase appetite and make the body relatively resistant to insulin," Patel says. Both obesity and insulin resistance are risk factors for heart disease.
Another possibility is that chronic sleep deprivation might inflame the linings of arteries. Scientists have found that even a few nights of sleep loss raises levels of inflammation markers.
Whatever the explanation, Patel says he's impressed that the heart's need for sleep is so clear.
"That's one of the most notable findings of this study — that the association is so strong," he says. "They found that for every additional hour that people slept, their risk of having a coronary calcification went down by a third, which is a huge effect."
It's a bigger benefit than doctors typically see when they prescribe a drug that lowers blood pressure.
Lauderdale says more work is needed to pursue this lead, so it's too early to say that people should sleep more to lower their risk of heart disease.
"However, this study does add to mounting evidence that there are subtle but potentially important health consequences of routinely sleeping very short hours — say, less than five hours a night," she says.
She says it's wise to sleep at least six or seven hours a night. That's pretty benign advice, she says, which is "why I'm feeling OK about giving it."
It's advice she tries to live by.