Surveys have long shown that women are more likely to be “morning larks” than “night owls.”
In other words, women’s natural tendency is to go to bed early and wake up early, while men tend to prefer the opposite: staying up late at night and sleeping in late in the morning.
The reason for this general pattern has not been clear. But new research from the Harvard University-affiliated Brigham and Women’s Hospital in Boston has found a possible explanation: Women’s circadian (24-hour) biological cycles tend run shorter than men’s.
The authors of the study, published earlier this month in the Proceedings of the National Academy of Sciences, believe this finding may also help explain why women are more prone to insomnia and waking up early than men — and why women find it easier to adjust to jet lag when they’re traveling eastward across time zones.
No time cues
For the study, researchers put 157 volunteers (52 women, 105 men), aged 18 to 74, in a sleep lab for two to six weeks. The lab had no windows or other external time cues, so the volunteers’ inner clocks were free to follow their natural circadian cycle. (We unconsciously use sunlight and other cues to reset our cycles every day.)
The circadian cycles for individual volunteers in the study ranged from about 23½ to 24½ hours. Age had no effect on the length. The average cycle for both men and women ran slightly over 24 hours, but — and here’s the key finding — the cycles of women averaged about 6 minutes less than those of the men. The women were also 2.5 times more likely to have cycles shorter than 24 hours.
OK. Six minutes may not sound like much. But, as the authors of the study point out, that six minutes translates into waking up about 30 minutes earlier. If women are not getting to bed early (a difficult thing to do in our current culture, when so much housework, child care and other tasks need to be accomplished in the evening, after a full day’s work), then women run the risk of becoming chronically sleep deprived — and perhaps depressed. Sleep deprivation is associated with a higher risk of depression.
Link to estrogen?
Why do women’s circadian cycles tend to run shorter than men’s? It may be related to higher estrogen levels, say the study’s authors, although no difference in cycle lengths was noticed between pre- and postmenopausal women. (Estrogen levels decrease after menopause.) Perhaps, add the authors, the shorter cycles are “set” in women by their exposure to higher estrogen levels during early development. That early exposure may permanently alter the central circadian pacemaker, which is believed to reside in the brain’s hypothalamus.
What can you do to about this? Not much. Research has shown that whether we are a “morning” or an “evening” person is a permanent trait, and not something we can change. (And, yes, not all men and women fit the general pattern, which may put a kibosh on the estrogen theory. Some women love to stay up until the wee hours of the night and some men jump out of bed early every morning, before the alarm goes off.)
If you’re a woman with a fast-running circadian cycle, however, you can take steps to help ensure you get enough sleep. Try to get to bed at a time that’s natural to your sleep cycle. And make sure your bedroom is completely dark.
Not that there aren’t some advantages to a fast-running circadian cycle. One is overcoming jet lag when traveling east — say, to Europe. Having a shorter internal clock makes it easier to reset it to an earlier time zone, studies have found.
