Seven Hours' Sleep "the safest"

Posted by Richard Linney on Feb 15, 2002 at 07:45 (

People who sleep for eight hours or more every night have a higher death rate than those who average six to seven hours, according to a new US study. But many sleep experts are sceptical about the findings.

A team led by Daniel Kripke at University of California, San Diego surveyed 1.1 million people aged between 30 and 102. In 1982, the participants were asked about their sleep duration, frequency of insomnia and sleeping pill use. They were then reviewed again in 1988. The researchers found that those who slept for an average of eight hours or more, or less than four hours, each night had significantly increased mortality rates.

"People who sleep for eight hours are 13 to 15 percent more likely to die within six years than those who sleep seven hours. In fact, even sleeping for five hours was less risky than eight," Kripke says.

The study took into account factors such as age, diet, previous health problems and factors such as smoking. About half of the group studied slept for eight hours, although the average sleep time in the western world is now 6.5 hours.

Protective insomnia

The findings contradict the widely held view that sleep deprivation and insomnia are dangerous to health and that increased sleep leads to increased longevity. "The important message is that it is very safe to sleep five, six or seven hours - sleeping for longer is certainly not risk free. In fact, insomnia can be seen as protective," Kripke told New Scientist.

But Orfeu Buxton at the University of Chicago sleep research centre says: "They have used subjective measures for sleep that are a poor surrogate for actual sleep measures. Every other study I know of shows that sleeping longer is better for you."

Kripke admits that he has no clear explanation for the findings. But, he says: "Sleep length might affect cytokines, which are transmitters that affect inflammation, and this might somehow influence mortality." People who sleep longer may also be an increased risk of sleep apnoea, when breathing stops during sleep, he suggests.

Bruce Carnes of the University of Chicago says it is not possible to assign simple mortality risks to different sleep durations. "Everybody is unique and has a different ideal sleeping time," he says.

Sleeping pills

Kripke's research also found an increased risk of mortality from using sleeping pills.

Half of all the women and over 70 per cent of men studied by Kripke's team reported never having suffered insomnia, but 4.3% of women and 2.6% of men reported experiencing insomnia 10 or more times per month.

People who reported occasional bouts of insomnia did not have an increased mortality rate. But those sufferers who regularly took sleeping pills were more likely to die sooner.

Journal reference: Archives of General Psychiatry (vol 59, p131)

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