Boston, MA – Researchers from the Division of Sleep Medicine at Brigham and Women’s Hospital (BWH) led by Dr. Charles Czeisler and Dr. Jeanne Duffy, studying differences in circadian rhythms of melatonin and temperature between men and women, found that 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. The findings remained consistent for both young and older men and women.

The findings were published online today in the Proceedings of the National Academies of Science.

The researchers measured the cycle length of the circadian timing system of 52 women and 105 men 18-74 years of age. The study participants were each studied for 2-6 weeks on a special schedule in an environment shielded from external time cues to assess the cycle time of their brain’s circadian clock. The researchers found that the natural circadian cycle of individual participants ranged from about 23 ½ to 24 ½ hours and age did not have an effect on the duration of the cycle. They 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 the men, and women were 2 ½ times more likely than men to have cycles shorter than 24 hours. This could lead to women having earlier waking times and might explain the higher prevalence of insomnia among women.

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

Dr. Jeanne Duffy, lead author of the study and a researcher in the Division of Sleep Medicine at BWH added, “The shorter circadian cycle in women 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.”

The study was supported in part by the National Institutes for Health, the National Aeronautics and Space Administration and the National Space Biomedical Research Institute.