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Watching the Clock

Circadian rhythms differ between sexes

The brain’s circadian clock cycles at a faster pace in women, which may help account for why women tend to wake earlier and are more likely than men to experience sleep maintenance insomnia, according to research at Brigham and Women’s Hospital. The findings, which remained consistent for both younger and older men and women, were published May 2 online in the Proceedings of the National Academies of Science.Nick Veasey/Getty

The researchers measured the cycle length of the circadian timing system of 52 women and 105 men aged 18 to 74. The study participants were each studied for two to six weeks on a special schedule in an environment shielded from external time cues to assess the cycle time of his or her brain&rsqou;s circadian clock. The researchers found that the natural circadian cycle of individual participants ranged from about 23–and–a–half hours to 24–and–a–half hours, and that age did not have an effect on the duration of the cycle. The researchers also found that the circadian cycle length in both men and women averaged slightly longer than 24 hours, but the circadian cycle of the women averaged some six minutes shorter than that of the men, and women were two and a half times more likely than men to have cycles shorter than 24 hours.

“The findings are important in that they show a true sex difference in the brain’s circadian clock between people,” says Charles Czeisler, chief of the Division of Sleep Medicine at Brigham and Women’s Hospital. “Knowing this can help in tailoring sleep therapies based on sex.”

“The shorter circadian cycle in women,” says Jeanne Duffy, lead author of the study and an HMS associate professor of medicine, “may be due to higher levels of estrogen, although we need to do further research to understand why women’s circadian cycles tend to be shorter than those in men.”

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