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小時睡眠才可以保持身體健康——這條“真理”已不再適用，因為越來越多的證據表明睡覺時間少于8小時可能會更有益于健康。

    三次大規模的研究發現每晚睡7小時左右的人壽命最長，其中最近的一次研究結果在二月份的《睡眠》期刊上發表。

    盡管如此，許多研究睡眠的科學家說很多成年人的休息時間太少，這會導致健康危機。

    在日本研究者的最新研究報告中，他們在十年時間里密切觀察了十萬四千零十位成年人。一開始，參與者回答了有關睡眠方式、健康狀況、精神狀況和生活習慣的問卷調查，這些因素都可能影響他們的壽命。

    分析了以上這些因素之后，發現平均每天睡七小時的成年人的死亡率最低。令人驚訝的是，如果睡眠的時間更少，甚至每晚只睡4小時，男性的死亡率并沒有明顯上升。而如果女性的平均睡眠小于4小時，她們的壽命則會縮短。但是睡眠時間超過7小時的成年人，特別是女性，很可能在10年內去世。

    加州大學圣地亞哥分校醫學院的睡眠研究者、精神病醫生丹尼爾·克里普卡說，另外兩個已經發表的大規模研究和許多小型研究都得出了類似的結論。

    他在《睡眠》雜志上發表評論說，醫生不應該要求所有病人都至少睡8小時。

    克里普卡說，黑暗或其他由長時間睡眠引起的未知生理反應會導致荷爾蒙發生變化，這會影響壽命。

    但是，睡眠不足的人可能會受到其他的不良影響。在他短暫的研究中，那些睡眠時間在4到5個半小時的人在有關記憶力、清晰的思考問題和集中注意力方面的測試中表現很差。賓夕法尼亞大學醫學院的大衛·丁格斯說：“持續時間越長，他們的情況就越糟。”他說，睡7個小時的成年人表現的最好。

    其他小型研究發現睡眠時間小于6小時的成年人得糖尿病和肥胖癥的危險很大，缺少睡眠也會引起撞車，丁格斯說。

    “人們應該睡足他們需要休息的時間，”克里普卡說。他還補充說，沒有證據證明縮短睡眠時間能夠延長壽命。睡眠的需求一部分取決于基因，也可能取決于其他一些因素，這些因素還影響生命的長短。