The truism that all adults need at least eight hours of sleep a night for good health should be put to rest by mounting evidence that less may be better.

People who sleep about seven hours a night live the longest, three huge studies have found, the newest out in the February issue of the journal SLEEP.

Still, many sleep experts say lots of adults get too little rest, and that can lead to dangerous health problems.

In the latest report from Japanese researchers, 104,010 adults were followed for about 10 years. At the start, the participants answered questionnaires about their sleep patterns, and about their health, mental health and lifestyle habits, which also can affect survival.

After accounting¹ for all of these factors, adults getting an average of seven hours had the lowest death rates. Surprisingly, less sleep, even as little as four hours a night, didn't significantly increase deaths for men and only lowered survival for women if they averaged less than four hours. But adults who slept longer than seven hours, particularly women, were more likely to die during the 10 years.

Two other major published studies and a dozen smaller ones came to similar conclusions, says psychiatrist² Daniel Kripke, a sleep researcher at the University of California-San Diego School of Medicine.

Doctors shouldn't tell all of their patients to get at least eight hours of sleep, he says in an editorial in the journal.

Hormonal³ changes triggered by darkness or other unknown biological effects from long sleep could be affecting survival, Kripke says.

But short sleepers⁴ may suffer other bad effects. In his brief studies, those sleeping four to five and a half hours did poorly on tests that measure memory, clear thinking and the ability to pay attention, "and they did progressively worse as the week went on," says David Dinges of the University of Pennsylvania School of Medicine. Adults who slept about seven hours performed best, he says.

Other small studies have found adults who sleep less than six hours may be at higher risk for diabetes⁵ and obesity⁶. And sleep deprivation⁷ also causes car crashes, Dinges says.

"People should get as many hours sleep as they need to feel rested," Kripke says, adding that there's no proof that shortening sleep will lengthen⁸ life. Sleep need is partly genetic⁹ and may be determined¹⁰ by other factors that also influence life span, he says.