Six or seven hours sleep is better than eight
A study that included more than a million participants found people who sleep eight hours or more died younger.
Those who only managed four or less hours in the land of nod were similarly affected but six or seven hours a night was found to be conducive to a longer life.
The research, carried out by scientists at the University of California, showed a clear association between long duration sleep and high mortality rates.
The report's author Dr Daniel Kripke, a professor of psychiatry, 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.
"Individuals who now average six-and-a-half hours of sleep a night can be reassured this is a safe amount of sleep.
"From a health standpoint there is no reason to sleep longer."
The findings were being discussed at the annual meeting of the American Association for the Advancement of Science in Boston, Massachusetts.
Margaret Thatcher famously managed to function on just four hours' sleep a night.
Sleeping tablet danger
That may be too little, but people who managed as little as five hours a night lived longer than those who got more than eight hours.
The best survival rates were among those who slept seven hours a night, according a report published in the Archives of General Psychiatry.
Professor Jim Horne at Loughborough University's Sleep Research Centre said people who advocate the benefits of lots of sleep are misguided.
He said: "A lot of people can be reassured that six or seven hours sleep is okay.
"The acid test for enough sleep is whether you are sleepy or alert throughout the day.
"If you are alert, then your sleep is probably adequate.
"However, nine hour sleepers shouldn't think they're going to die early."
The US study showed that a group sleeping eight hours were 12% more likely to die within the six year period covered by the study than those sleeping seven hours.
Between 1982 and 1988, a total of 5.1% of the women taking part and 9.4% of the men had died.
The study, involving 1.1million people was the first large-scale population study of sleep to take into account variables such as age, diet, exercise, previous health problems and risk factors such as smoking.
The study also found that occasional bouts of insomnia were not linked to higher death rates.
However, it concluded that people who take sleeping pills are more likely to die earlier.