The secret to a long life among women may mean just enough sleep - less than five hours a night is probably not enough while eight hours is probably too much.
A team of scientists, headed by Daniel F. Kripke, professor emeritus of psychiatry at the University of California, San Diego School of Medicine, US revisited original research conducted between 1995 and 1999.
In the previous study, part of the Women's Health Initiative, Kripke and colleagues had monitored 459 women living in San Diego (aged from 50 to 81 years).
Fourteen years later, they returned to see who was still alive and well, reports the journal
Of the original participants, 444 were located and evaluated. Eighty-six women had died. Previous studies, based upon questionnaires of people's sleep habits, had posited that sleeping 6.5 to 7.5 hours per night was associated with best survival.
Kripke and colleagues, whose 1990s research had used wrist activity monitors to record sleep durations, essentially confirmed those findings, but with a twist, according to a California statement.
"The surprise was that when sleep was measured objectively, the best survival was observed among women who slept 5 to 6.5 hours," Kripke said.
"Women who slept less than five hours a night or more than 6.5 hours were less likely to be alive at the 14-year follow-up."
Kripke said the study should allay some people's fears that they're not getting enough sleep.
"This means that women who sleep as little as five to six-and-a-half hours have nothing to worry about since that amount of sleep is evidently consistent with excellent survival."