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ANN ARBOR ? Working with an unusual colony of South American rodents called degus (day-goos), University of Michigan researchers have discovered striking sex differences in how quickly the creatures re-set their biological clocks in response to changes in light and social contact.

The discovery could lead to different ways of helping men and women who suffer from Seasonal Affective Disorder (SAD), jet lag, shift work problems , and other disruptions in their circadian rhythms.

Male degus shift their body temperature and activity cycles much quicker than females when exposed to light cues alone. Females resynchronize their internal clocks much faster, however, if they’re also getting social cues from other females who’ve already made the changes. For the research, U-M biopsychologist Theresa M. Lee and graduate students Marcia Governale and Namni Goel conducted the series of experiments with Octodon degus, a small diurnal rodent well-suited for research on circadian, or daily, rhythms. They have body temperature and physical activity cycles similar to people. They’re also social animals with individual as well as male-female differences in adapting to circadian changes. “We know that the circadian rhythms of men and women differ,” says Lee, an associate professor. “But there’s very little good research on the reasons. Our work with degus suggests that males and females may have different levels of sensitivity to different kinds of environmental synchronizing cues.”

For one set of studies, Lee and associates analyzed how long it took male and female degus to readjust to six-hour advances and delays in the 24-hour light-dark cycle. When the animals were housed alone, males readjusted faster than females. When females were housed with other females who had already adapted to the new light-dark cycle, however, their recovery rate increased by 30 to 40 percent.

Male “phase shifters” showed no sensitivity to cues provided by animals of either sex, a finding that might not surprise misanthropists.

“In most species of vertebrates, the light-dark cycle is the most important environmental cue for re-setting the circadian clock,” notes Governale. “But there are other cues as well, many of them involving social behavior. Birds resynchronize activity cued by song alone, with light and temperature held constant. Bats are influenced by the activities of other bats.”

To find out what it is about the social cues female degus give each other that enhances their ability to shift phases, Governale conducted a series of studies slightly different from the first studies, in which the animals had either been housed alone or housed in the same cage, separated from each other by a screen.

Governale put female degus who were on different circadian cycles into separate environmental chambers linked by an air hose that pumped air from the chamber occupied by an already-adjusted “donor” into the box of the female phase shifter. Female degus who could smell another female already on the new cycle changed to a new cycle themselves 39 percent faster than those reacting to light cues alone.

It’s a far cry from degus to people, Lee cautions. Still, the line of research suggests a way to enhance the effectiveness of bright light treatment for women who find themselves becoming sleepier, hungrier and more depressed as the winter solstice approaches on December 21.

Don’t just go outside, or keep the lights turned on while you’re in. Seek out the company of women friends who are not chronically sleepy, hungry or depressed. They may be way ahead of you.

University of Michigan