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 accounting 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 psychiatrist 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.

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

But short sleepers 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 diabetes and obesity. And sleep deprivation 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 lengthen life. Sleep need is partly genetic and may be determined by other factors that also influence life span, he says.

成年人每天至少需要8小时睡眠才可以保持身体健康——这条“真理”已不再适用，因为越来越多的证据表明睡觉时间少于8小时可能会更有益于健康。

三次大规模的研究发现每晚睡7小时左右的人寿命最长，其中最近的一次研究结果在二月份的《睡眠》期刊上发表。

尽管如此，许多研究睡眠的科学家说很多成年人的休息时间太少，这会导致健康危机。

在日本研究者的最新研究报告中，他们在十年时间里密切观察了十万四千零十位成年人。一开始，参与者回答了有关睡眠方式、健康状况、精神状况和生活习惯的问卷调查，这些因素都可能影响他们的寿命。

分析了以上这些因素之后，发现平均每天睡七小时的成年人的死亡率最低。令人惊讶的是，如果睡眠的时间更少，甚至每晚只睡4小时，男性的死亡率并没有明显上升。而如果女性的平均睡眠小于4小时，她们的寿命则会缩短。但是睡眠时间超过7小时的成年人，特别是女性，很可能在10年内去世。

加州大学圣地亚哥分校医学院的睡眠研究者、精神病医生丹尼尔·克里普卡说，另外两个已经发表的大规模研究和许多小型研究都得出了类似的结论。

他在《睡眠》杂志上发表评论说，医生不应该要求所有病人都至少睡8小时。

克里普卡说，黑暗或其他由长时间睡眠引起的未知生理反应会导致荷尔蒙发生变化，这会影响寿命。

但是，睡眠不足的人可能会受到其他的不良影响。在他短暂的研究中，那些睡眠时间在4到5个半小时的人在有关记忆力、清晰的思考问题和集中注意力方面的测试中表现很差。宾夕法尼亚大学医学院的大卫·丁格斯说：“持续时间越长，他们的情况就越糟。”他说，睡7个小时的成年人表现的最好。

其他小型研究发现睡眠时间小于6小时的成年人得糖尿病和肥胖症的危险很大，缺少睡眠也会引起撞车，丁格斯说。

“人们应该睡足他们需要休息的时间，”克里普卡说。他还补充说，没有证据证明缩短睡眠时间能够延长寿命。睡眠的需求一部分取决于基因，也可能取决于其他一些因素，这些因素还影响生命的长短。