Contrary to popular belief, people who sleep six to seven hours a night live longer, and those who sleep eight hours or more die younger, according to the latest study ever conducted on the subject. The study, which has tracked the sleeping habits of 1.1 million Americans for six years, weakens the advice of many sleep doctors who have long recommended that people get eight or nine hours of sleep every night.

    “There's an old idea that people should sleep eight hours a night, which has no more scientific basis than the gold at the end of the rainbow,” said Daniel Kripke, professor of psychiatry（精神病学）at the University of California at San Diego who led the study published in a recent copy of JAMA Psychiatry.

    The study was not designed to answer why sleeping longer may be harmful or whether people could extend their lifespan by sleeping less.

    But Kripke said it was possible that people who slept longer tended to suffer from short-term absence of sleeping, a condition where weak breathing puts stress on the heart and brain. He also stressed that the need for sleep was similar to that for food, where getting less than people want may be better for them.

    The study quickly caused warnings and criticism（批评）, with some sleep experts saying that the main problem of America's sleep habits was deprivation（剥夺）, not sleeping too much.

    “None of this says sleep kills people,” said Daniel Buysse, a psychiatrist at the university of Pittsburgh Medical Center.

    “You should sleep as much as you need to feel awake, alert and attentive the next day,” Buysse added. “I'm much more concerned about people short-changing themselves on sleep than people sleeping too long.”

    “Sleeplessness produces a variety of health consequences that were not measured in the study,” critics said.