You should get no less than four, no more than eight, but six or seven hours of nightly rest is preferable if you want to live longer, a new study says.
Researchers at the University of California, San Diego School of Medicine and the American Cancer Society surveyed 1.1 million adults between the ages of 30 and 102. They found that individuals who sleep eight hours or more hours a night, or less than four, have a significantly higher mortality rate than those who get an average of six to seven hours of shut-eye. Those who rise after seven hours have the best odds.
The researchers also found that people who reported occasional insomnia had no elevated risk, while those who said they took sleeping pills did have an increased mortality rate.
The findings appear in the February issue of the Archives of General Psychiatry.
Dr. Daniel Kripke, first author of the study and a psychiatry professor at the university, insists that the findings should serve to reassure light sleepers and not frighten heavy sleepers.
"The emphasis is not on the risk, but on the safety," he says. "It's really quite safe to sleep only five, six or seven hours. You really don't need to sleep eight hours. Since the average American only sleeps six and a half hours, the good message of this study is that it's perfectly all right."
Although earlier studies have come to similar conclusions, no other study managed to make such a fine distinction between seven and eight hours of shut-eye. This is also the first study to take into account 32 different variables, including age, diet, exercise and smoking history.
The study relied on surveys filled out in 1982 by 1.1 million adults as part of the American Cancer Society's Cancer Prevention Study II. The participants were revisited six years later, to determine who was still alive and who had died. It has taken the remaining 14 years to input and analyze the data.
"The raw results are very similar to those of the first study [completed in the 1960s]," says Kripke. "What we were able to do in the new study, though it took us years and years, was to control simultaneously for 32 risk factors. It was not until we got a more recent Pentium [computer chip] that we could really do the computation for a million people looking at 32 variables simultaneously."
What the research revealed was: Those who got more than eight hours or less than four or five hours sleep had at least a 15 percent increased risk of dying within the six-year period studied compared to those who slept the ideal of seven hours. Also, those who got eight hours of sleep were 12 percent more likely to die within the six-year period than those who slept seven hours.
But Kripke says there's no real cause for concern.
The vast majority of people sleep in the range of five to nine hours a night. "There are certain mortality distinctions within that range, but they're not huge," Kripke says. "We're talking about 10 or 20 percent increased risk, so they're modest."
Kripke himself sleeps a solid eight hours a night and isn't worried. "The amount of risk which is involved in sleeping eight hours rather than seven is pretty small," he says.
It's possible that longer sleep patterns might be associated with sleep apnea, which, in turn, is associated with hypertension and heart disease, says Dr. Milton Erman, adjunct professor in the division of neuropharmacology at Scripps Research Institute in La Jolla, California. "This might explain why this population has greater mortality," he adds.
Two shorter-acting sleeping pills were also not yet on the market when the study data was being compiled, which might impact the findings. It also wasn't clear whether the respondents who said they were taking prescription drugs might have added confounding conditions.
Even with all the Pentium-processed number crunching, researchers still don't know why the sleep disparities exist. "We can only speculate but we don't really know," says Kripke.
"The bottom line is getting the rest we need," Erman adds. "Too little sleep may be harmful. The best indicator is being able to feel and function well during the day."
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