Sleeping People who sleep for more than eight hours a night do not live as long as those who sleep for six hours, according to the biggest study yet into sleep patterns and mortality (死亡率). Scientists have no explanation for the findings and do not know if they mean people who like a lie-in (睡懒觉) can extend their lives by sleeping less. Although it is a common belief that sleeping for at least eight hours a night is vital for health and well-being, the six-year study involving more than 1.1 million Americans older than 30 found that those who slept for less than eight hours were far from doing themselves any long-term harm. 'Individuals who now average 6.5 hours of sleep a night can be reassured that this is a safe amount of sleep. From a health standpoint, there is no reason to sleep longer,' said Daniel Kripke, a professor of psychiatry (精神病学) at the University of California, San Diego. Dr. Kripke said, 'We don't know if long sleep periods lead to death. Additional studies are needed to determine if setting your alarm clock earlier will actually improve your health.' The scientists, who were funded by the American Cancer Society, found that the best survival rates were among the men and women who slept for seven hours a night. Those who slept for eight hours were 12 per cent more likely to die during the six-year period of the study, when other factors such as diet and smoking were taken into account. Even those who spent a mere five hours a night in bed lived longer than those who slept eight or more hours. However, an increasing death rate was found among those who slept for less than five hours. Dr. Kripke said, 'Previous sleep studies have indicated that both short- and long-duration (持续时间) sleep had higher mortality rates. However, none of those studies were large enough to distinguish the difference between seven and eight hours a night, until now.' More than one million Americans participated in the six-year study.