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RENEE MONTAGNE, Host:

Here's the latest on your heart. Hearts need a certain amount of sleep every night to stay healthy. That's from a study in this week's Journal of the American Medical Association. And the link between sleep and heart health is stronger than researchers suspected, as NPR's Richard Knox reports.

RICHARD KNOX: 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 notoriously unreliable.

University of Chicago researchers decided to look more carefully. They outfitted 500 volunteers in their early 40s with a device that they wore on their wrists. It measured their movements day and night. Diane Lauderdale says when people are asleep the device registers a characteristic pattern of movement.

DIANE LAUDERDALE: It's not actually no movement. There is slight motion during sleep. It's not - it doesn't look the same as if you just take the watch off and put it on the counter.

KNOX: That gave Lauderdale's group an objective readout of how much people actually sleep. To measure their underlying heart health, the researchers did CT scans of their coronary arteries. The scans can reveal telltale deposits called calcifications. Those are 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.

LAUDERDALE: Twenty-seven percent of them developed coronary artery calcification over the five years of follow-up, whereas the persons who slept seven hours or more on average, only six percent developed coronary artery calcification.

KNOX: The sleep-deprived people had four and a half times the risk of heart disease after the researchers subtracted out the effects of other known coronary risk factors such as 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 such stress hormone, or maybe it's related to blood pressure.

LAUDERDALE: It's possible that people who are sleeping longer hours have a deeper or more sustained blood pressure dip during the night.

KNOX: Doctor Sanjay Patel of Case Western Reserve Medical School thinks sleep might affect hormones that regulate body weight, appetite, and metabolism.

SANJAY PATEL: 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.

KNOX: Another possibility, chronic sleep deprivation might inflame the linings of arteries. Scientists have found that even a few nights of sleep-loss raises inflammation levels. Whatever the explanation, Patel says he's impressed that the heart's need for sleep is so clear.

PATEL: That's one of the most notable findings of this is that the association is so strong. They found that for every additional hour that people slept, their risk of having coronary calcification went down by a third, which is a huge effect.

KNOX: A bigger benefit than doctors typically see from drugs that lower blood pressure. Lauderdale says more work is needed to pursue this lead.

LAUDERDALE: 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.

KNOX: She says it'd be wise to sleep at least six or seven hours a night. After all, that's pretty benign advice.

LAUDERDALE: That is why I'm feeling OK about giving it.

(SOUNDBITE OF LAUGHTER)

KNOX: And, it's a rule she tries to live by. Richard Knox, NPR News.

MONTAGNE: It's NPR News.

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