People with poor sleep habits may be at higher risk for early signs of heart disease than those who get adequate, good quality sleep, a study suggests.

Getting either too little or too much sleep can increase the risk of heart diseases, researchers in South Korea say.

In a study of over 47,000 young and middle-aged adults, elevated levels of calcium in arterial wall and the beginning of hardening of the arteries — two risk factors for heart disease — were found in men and women who either slept for nine or more hours a night, considered "long sleepers," or fewer than five hours as "short sleepers," the researchers report in a journal of the American Heart Association.

The average amount of sleep reported by survey participants was 6.4 hours of sleep nightly, with 84 percent of the participants rating their sleep quality as "good."

Both short and long sleepers had a higher content of calcium in their coronary arteries than did those who had the average amount of good sleep, the study showed.

"The calcium score obtained by computerized tomography scan is a very good measure of calcium buildup in the coronary arteries reflecting coronary atherosclerosis," says Dr. Chan-Won Kim of Sungkyunkwan University School of Medicine in Seoul. "The higher the coronary calcium score, the greater the risks of having a heart attack in the future."

The study's co-leader, Kanbuk Samsun Hospital's Yoosoo Chang, stated that they observed similar patterns when measuring stiffness of the arteries and that adults who sleep poorly have arteries that are stiffer than individuals who enjoy good quality sleep or sleep that lasts seven hours each day.

The study findings add to previous research that found people getting less than six hours of sleep at night were more likely to be obese and suffer from high blood pressure, diabetes and high levels of cholesterol.

The findings emphasize the importance of getting a good night's sleep, the researchers say.

"Overall, we saw the lowest levels of vascular disease in adults sleeping seven hours a day and reporting good sleep quality," says Chang.

Kim agrees, although he acknowledged the study shows an association between poor sleep and heart disease, not a proven cause and effect.

"It is still not clear if inadequate sleep is the cause or the consequence of ill health," he says, but noted that good sleep habits, including avoiding use of electronic media at bedtime, should be part of a healthy lifestyle.

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