Sleeping longer - like getting eight hours or more a night - could shorten your life, according to a study of the nighttime habits of more than a million Americans.
Researchers at the University of California at San Diego found in a six-year study of adults aged 30 to 102 that people who sleep five to seven hours a night live longer than those sleeping eight hours or more.
The study found little, if any, connection between death rates and insomnia, which it said patients commonly complain of even when their sleep duration is within normal ranges, but said sleeping pills actually increase the risk of death.
"People who sleep five, six or seven hours have nothing to worry about. There is no evidence that people need eight hours of sleep ... the only basis for that is it's what grandma used to say," Dr. Daniel Kripke, a UCSD professor of psychiatry and the study's lead author, said in an interview.
He said people who take sleeping pills were shown to have a slightly higher death rate than others, but because the data was collected in the 1980s, the results are not necessarily reflective of newer types of sleeping pills.
Kripke, an eight-hour-a-night sleeper, said the average American gets six-and-a-half hours of shut-eye, which is just fine. "Neither in terms of health or survival is there any evidence that eight hours of sleep is better than six or seven. Nor is there any evidence that longer sleepers are rich or have more fun," Kripke said.
Sleep experts said the research, though provocative, has several flaws. The study was not actually designed to look at sleep's effect on longevity. It relied on patients' recollections of their sleep habits and did not ask if they took naps. It did not look at the quality of people's sleep or whether they felt drowsy all day.
Dr. Phyllis Zee, director of Northwestern Memorial Hospital's sleep disorders center, said the results probably do not reflect the general population because participants were not randomly selected but were mainly friends and relatives of volunteers for the American Cancer Society, which collected the data as part of a 1982 survey on cancer risks.
Zee said it is possible that participants who got little sleep or slept eight hours or more had medical problems that would explain their increased death rate.
Dr. Kripke said more research is needed to determine whether sleeping longer should be added to the growing list of one-time pleasures - like smoking and alcohol - now deemed hazardous to your health.
"Long sleep is a risk factor for cancer as well as heart disease and stroke, but we don't really know why," he said.
The research, which addressed sleep issues as part of a broader cancer prevention study done with the American Cancer Society, will be published in Friday's issue of the Archives of General Psychiatry.
The best survival rates were found for people who slumbered seven hours a night -- about 32 percent of women in the study and 34 percnt of the men fell into this category. Compared to them, those sleeping eight hours were 12 percent more likely to die within the six-year period that the study was conducted, 1982 to 1988.
People with as little as five hours of sleep lived longer than participants with eight hours or more per night but compared to the seven hour group, their mortality risk was seven percent higher for women and 11 percent for men. Study participants sleeping less than four hours a night died sooner than those averaging six to seven hours.
Heart disease was the most common cause of death, followed by stroke and cancer.
The study was designed to account for dozens of variables, such as age, health problems and education, that would affect life span. The mean age was 57 for women and 58 for men.
Although the study was conducted from 1982 to 1988, the results have not been available until recently due to technological constraints and the length of time required to analyze the vast amount and variety of data.
Kripke likened the habit of lingering in bed to overeating. "Everyone knows that eating too much is bad for you. If you eat a little bit less than you desire, you will live longer," he said. "It is also true that if we look across the mammalian kingdom, the short sleepers live longer."
He said some sleep loss actually acts as an antidepressant.
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