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A new study that might keep people awake at night has found a mysterious link between sleeping less and living longer.

Researchers discovered that all other factors of diet, age, health and lifestyle being equal, those who slept eight hours a night were 12 per cent more likely to die within a six-year period than those who slept seven hours.

Even those sleeping as little as five hours a night lived longer than those who slept eight hours or more. Those who caught between six and seven hours had the lowest death rate.

People in the study, which included more than a million participants, died of common, natural killers, like heart disease, strokes and cancers.

The results have stumped the researchers, who cannot explain the association between an eight-hour sleep duration and higher death rates.

They have also irked others, who worry that today's society, already characterized by sleep deprivation, needs no prompting to enjoy less pillow time.

But the lead author of the study, published today in the Archives of General Psychiatry, insists the matter demands further investigation.

"Additional studies are needed to determine if setting your alarm clock earlier will actually improve your health," said Daniel Kripke, a sleep research specialist and psychiatry professor at the University of California in San Diego.

"It is quite possible that just as we restrict our diets for better health, we may have to restrict our sleep a little and not sleep all we can."

What is clear, Dr. Kripke said, is that adults do not need eight hours a night to be healthy.

"It's very safe to sleep five, six or seven hours a night," he said. "The idea that we need eight hours is a widely circulated belief -- even sleep researchers believe it -- though there's no evidence."

It has long been known that people who sleep excessively, more than 10 hours nightly, and those who snooze fewer than three or four hours, die sooner than those with average sleeping habits.

In part, this is because extreme sleeping habits are usually a symptom of some other underlying health problem, such as depression, sleep apnea, obesity, or anorexia, said Colin Shapiro, director of the sleep and alertness clinic at the Toronto Western Hospital.

This latest study confirmed that sleeping less than four hours also increases the risk of death. Yet the notion that an eight-hour sleep constitutes the far end of a normal sleeping range is new, and to some, controversial.

"I can tell you a lot of people who feel good after eight or 8½ hours sleep," said Meir Kryger, director of the sleep disorders centre at St. Boniface General Hospital and professor of internal medicine at the University of Manitoba.

"Telling people they are sleeping too much or too little based on a population study is like saying someone is too tall or too short, what can you really do about it?"

No one disputes, for example, that children require significantly more sleep than adults and that adults lose their ability to sleep deeply as they age.

Dr. Shapiro pointed out that people's descriptions of their sleeping habits are also highly subjective, and time spent in bed is not the same as time spent sleeping.

According to Statistics Canada, people are sleeping an average of seven hours a night, an hour less than they did 25 years ago.

Dr. Kripke's research, which is part of a larger project looking at risk factors for cancer, is the first large-scale population study of sleep to consider such variables as age, diet, exercise, previous health problems and smoking.

Between 1982 and 1988, the researchers followed 1.1 million men and women between the ages of 30 and 102. But they said the results were not available until recently due to the amount of time required to analyze the mountains of data.

Dr. Kripke speculated that one factor to be explored in the relationship between sleep times and longevity is the activity of cytokines.

Cytokines, proteins that can help orchestrate immune-system cells against an invader, are released in the brain during sleep and can actually induce sleep.

But whether sleeping too much raises them to harmful levels, is not clear, Dr. Kripke said.