It's no secret that there are health risks associated with not getting enough sleep. Studies have shown that people who sleep less than six hours a night over long periods of time have increased risks of obesity, weakened immune systems, high blood pressure and cholesterol, heart disease and stroke.

But there's also such a thing as too much sleep, and new research indicates it carries similar risks.

In a National Health and Nutrition Examination Survey, people over age 45 who reported sleeping more than eight hours a night were twice as likely to have chest pain known as angina and were 10 percent more likely to have coronary artery disease.

Other studies have linked sleeping more than eight hours a night with obesity, and doctors have long known of a link between depression and sleeping long hours.

A separate study, published this spring in the American Journal of Epidemiology, found that the percentage of people who report sleeping more than nine hours a night has actually increased 9 percent in the United States since 1985.

Dr. Clete Kushida, medical director at the Stanford Sleep Medicine Center, said more research is needed to determine why certain health conditions are more common in people who sleep longer. But he points out that people naturally sleep less as they reach middle and old age. Nine or more hours of sleep is normal for adolescents, but it might be indicative of a problem if older people notice that their time asleep is increasing but still feel tired during the day.