truism that all adults need at least eight hours of sleep a night
for good health should be put to rest by mounting evidence that
less may be better.
People who sleep about seven hours a night live the longest,
three huge studies have found, the newest out in the February
issue of the journal SLEEP.
Still, many sleep experts say lots of adults get too little rest,
and that can lead to dangerous health problems.
In the latest report from Japanese researchers, 104,010 adults
were followed for about 10 years. At the start, the participants
answered questionnaires about their sleep patterns, and about
their health, mental health and lifestyle habits, which also can
After accounting for all of these factors, adults getting an
average of seven hours had the lowest death rates. Surprisingly,
less sleep, even as little as four hours a night, didn't significantly
increase deaths for men and only lowered survival for women if
they averaged less than four hours. But adults who slept longer
than seven hours, particularly women, were more likely to die
during the 10 years.
Two other major published studies and a dozen smaller ones came
to similar conclusions, says psychiatrist Daniel Kripke, a sleep
researcher at the University of California-San Diego School of
Doctors shouldn't tell all of their patients to get at least
eight hours of sleep, he says in an editorial in the journal.
Hormonal changes triggered by darkness or other unknown biological
effects from long sleep could be affecting survival, Kripke says.
But short sleepers may suffer other bad effects. In his brief
studies, those sleeping four to five and a half hours did poorly
on tests that measure memory, clear thinking and the ability to
pay attention, "and they did progressively worse as the week
went on," says David Dinges of the University of Pennsylvania
School of Medicine. Adults who slept about seven hours performed
best, he says.
Other small studies have found adults who sleep less than six
hours may be at higher risk for diabetes and obesity. And sleep
deprivation also causes car crashes, Dinges says.
"People should get as many hours sleep as they need to feel
rested," Kripke says, adding that there's no proof that shortening
sleep will lengthen life. Sleep need is partly genetic and may
be determined by other factors that also influence life span,