Women's Circadian Rhythm Beats Faster Than Men's

Studies have found that women report insomnia about 50 percent more often than men do. Their faster clocks may have something to do with it. 

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MICHELE NORRIS, host: We're going to hear now about a new study that confirms what some women have suspected about body clocks. The study shows that women run on a different and faster clock than men. It's published in the journal Proceedings of the National Academy of Sciences.

NPR's Richard Knox reports on some of the study's implications.

RICHARD KNOX: For years, surveys have shown that women more often identify themselves as early birds and more men call themselves night owls(夜猫子), although there are lots of exceptions. Studies have also found that women report insomnia(失眠) much more often than men. The new study, done on 157 people, may explain why.

At a sleep lab in Boston, volunteers spent up to eight weeks in a windowless room with no idea what time it was in the outside world. That allowed their inner clocks to drift according to each person's natural circadian rhythm(昼夜节律).

Jeanne Duffy of Brigham and Women's Hospital is a study author.

Dr. JEANNE DUFFY (Lead Author, Circadian Group, Division of Sleep Medicine): What we found was that the cycle length of the biological clock in women was shorter on average than it was in men. And the average difference was about six minutes.

KRULWICH: Now, she concedes that six minutes doesn't sound like much. But imagine you have a watch that runs six minutes too fast. If you didn't reset it, you'd get more and more out of sync(不同步) with the 24-hour cycle that we all have to live by.

We all have to set our biological clocks each day because none of us has an inner clock that's set to exactly 24 hours. If it was, we couldn't adjust to seasonal differences in night and day.

What the study found is that man's biological clocks tend to run slower and women's faster. In fact, some women - more than one in three - have inner clocks that run especially fast. They complete a daily cycle in less than 24 hours.

Ms. DUFFY: If your cycle length is shorter than 24 hours, you need evening light to keep you synchronized.

KNOX: And darkness in the morning. That's just the opposite of people whose inner clocks run on a cycle that's longer than 24 hours. They need a daily dose of morning light to reset their biological clocks.

Maybe the biggest implication of the study is that there may be millions of women who fight every day against their inner clock to do what they need to do.

Dr. CHARLES CZEISLER: (Professor of Sleep Medicine and Physician, Harvard Medical School): I definitely think that that's happening out there.

KNOX: That's Dr. Charles Czeisler, the study's senior author.

Dr. CZEISLER: I think many women are chronically sleep-deprived because their work obligations, they're taking care of children, helping them with homework whatever is happening that can only be done in the evening hours precludes many women from getting to bed early enough to get a full, sound night of sleep.

KNOX: A lot of women can identify with that. But Czeisler also says many women don't see their early-to-bed, early-to-rise tendency as a problem.

Dr. CZEISLER: There are many women who really enjoy the fact that they can get up at 4:00 or 5:00 in the morning. They have peace and quiet. Before the rest of the world wakes up, they can get an extraordinary amount done, and they wouldn't trade it for the world.

KNOX: In fact, he's married to one, Dr. Theresa Shanahan.

Dr. THERESA SHANAHAN: I am a morning person. It is a special time in the day that I enjoy, and get up and always have.

KNOX: Shanahan says she's at her most productive when the rest of her family is still in bed.

Dr. SHANAHAN: It's wonderful when I've accomplished four or five hours of work and the others are just waking up. Like, I've had a good jump on the day.

KNOX: Her husband likes to stay up late.

Dr. CZEISLER: I'm definitely an evening type.

KNOX: So how does it work, an early bird married to a night owl? You might think they don't spend much time together.

Dr. SHANAHAN: But if I'm more rested, I am much easier to communicate with. I have, you know, more energy, have a more positive, I think, outlook.

KNOX: So she says their time together is more likely to be quality time. Czeisler says when they first got married, he didn't realize they were circadian opposites.

Dr. CZEISLER: We were so much in love that, you know, we probably didn't even notice the time of day.

KNOX: But they've worked it out. Their 18th anniversary is later this month.

Richard Knox, NPR News, Boston.

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