Getting eight hours of sleep might not be your best bet after all, according to a new study.

Research published in Friday's issue of the Archives of General Psychiatry suggests adults live longer if they get six or seven hours of sleep each night, not eight.

SLEEP Sleep Statistics Sleep Myths How Much Sleep Do You Get? Do You Take Naps? STUDY Less Sleep Better? Stats from U. of California February 2002 study: Men's Stats Women's Stats RESOURCES Star Sleeper Campaign Sleep Tips For Children Why Sleep Is So Important

The research is based on a nationwide survey of 1.1 million adults. Those who slept eight hours a night were 12 percent more likely to die within six years than those who got 6.5 to 7.5 hours. The increased risk was more than 15 percent for those who reported getting more than 8.5 hours -- or less than about four hours -- nightly.

Even those with as little as five hours sleep lived longer than participants with eight hours or more per night, according to the study, which was conducted by researchers from the University of California, San Diego, School of Medicine and the American Cancer Society.

Although the study found the highest mortality rates with long-duration sleep, the study could not explain the causes or reasons for this association.

"We don't know if long sleep periods lead to death," said Dr. Daniel Kripke, a UCSD professor of psychiatry who specializes in sleep research. "Additional studies are needed to determine if setting your alarm clock earlier will actually improve your health."

But people who sleep an average of 6.5 hours a night can rest assured that it's a safe amount of sleep, Kripke said. "From a health standpoint, there is no reason to sleep longer."

The study also found that people who reported occasional bouts of insomnia did not have an increased mortality rate, but those individuals who took sleeping pills were more likely to die sooner.

"Insomnia is not synonymous with short sleep," the authors said. "Patients commonly complain of insomnia when their sleep durations are well within the range of people without sleep symptoms."

They added that physicians believe most patient complaints about "insomnia" are actually related to depression, rather than a diagnosis of insomnia.

Outside experts noted that the study did not ask such things as whether people took naps.

And the National Sleep Foundation said the population study wasn't randomized and didn't represent the entire population.

The foundation also criticized the study for trivializing the possible negative effects of insomnia and insufficient sleep, even if they are not statistically related to increased mortality.

Additional Resources:

• Archives of General Psychiatry
• National Sleep Foundation