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Never mind what your mother told you about needing a solid eight hours sleep.

The evidence is in and it's overwhelming: The ideal amount of sleep for an adult is seven hours nightly.

If you sleep more than that, or considerably less, you can expect to suffer from a number of health problems and die earlier, according to a large Japanese study published in today's edition of the medical journal Sleep.

Akiko Tamakoshi, a professor of preventive medicine at Nagano University, found that sleeping too much is far worse than sleeping too little.

His research also confirmed what most women already know, that men sleep more than women — 7.6 hours versus 7.1 hours on average. It also found that older adults sleep longer than younger ones, contradicting a popular belief that the elderly need very little sleep.

Those with the healthiest sleep habits tended to be married, and non-smokers. They were also the least likely to snore.

The research revealed that long sleepers tended to be single, less educated and suffer from depressive symptoms. The short-duration sleepers were characterized by high levels of stress and anxiety.

The study involved 110,792 subjects who were followed over a 10-year period.

Dr. Tamakoshi found that people who slept more than 10 hours nightly were 212 per cent more likely to die early. By comparison, those who slept less than four hours nightly saw their risk rise by about 83 per cent.

But sleeping even one hour more or less than the ideal seven hours raised mortality risks substantially — 35 per cent and 11 per cent respectively.

“The results falsify the widely circulated hypothesis that it is best to sleep at least eight hours,” Daniel Kripke, a professor of psychiatry at the University of California, San Diego, wrote in an editorial published in the same edition of Sleep.

He said what is really important about the new Japanese research is that it confirms the findings of two earlier American studies — the Nurses' Health Study, which followed more than 80,000 nurses, and the Cancer Prevention Study, which gathered data from almost 1.1 million people.

Dr. Kripke, who specializes in sleep research, said although the findings may surprise doctors and mothers alike, there is little doubt that people who sleep between 6.5 and 7.5 hours nightly have the lowest risk of premature mortality — even after other risk factors are taken into account.

“There is no reason to sleep longer,” he said.

The U.S. National Sleep Foundation, however, has repeatedly dismissed this type of research and is sticking with its recommendation that eight hours sleep is ideal.

“No one should change their sleep habits based on this information,” said Russell Rosenberg, director of the Northside Hospital Sleep Medicine Institute in Atlanta.

He said merely looking at the number of hours people sleep does not take into account the quality of sleep and individual differences. Dr. Rosenberg said there is a wealth of data showing that too little sleep hampers performance at work and school, increasing the risk of motor vehicle collisions and other accidents and weakens the immune system, and there is no comparable research showing that too much sleep is harmful.

Where the researchers agree is that it is unclear how or why the time a person spends sleeping affects their mortality. A leading theory is related to the link between sleep and the activity of cytokines. Cytokines are proteins that help regulate the immune system, and they are released by the brain during sleep. Sleeping too much, the theory goes, may damage the immune system, and leave people more susceptible to disease.

Dr. Kripke said that, because the mechanism is not known, it is premature to say that people should limit their hours of sleep the way they do their caloric intake to ensure better health. But he said on-going research should help answer that key unanswered question.

According to Statistics Canada, Canadians sleep, on average, seven hours nightly, one hour less than they did 25 years ago.