You may not be familiar with the term “circadian rhythm,” but you’re probably familiar with the idea of the body’s internal clock. A variety of biological processes in the body are set to a 24-hour cycle. Circadian rhythms, as these daily patterns are known, appear in both animals and plants. They’re present in sleep patterns, feeding routines, changes in body temperature, hormone production and many other cellular mechanisms. In humans, circadian rhythms drive when the body is at its peak physical condition and mental alertness, and there’s even evidence that these cycles can predict which team will win Monday Night Football games.
There are other, shorter cycles called ultradian rhythms that also influence physiology and brain function. These usually run in two- to six-hour cycles, but can be any length of time less than a day. Circadian rhythms are better known because they correspond to that primal unit of time, the 24-hour day, but ultradian rhythms are just as important in understanding what makes the body tick.
Brian Prendergast, PhD, professor in the department of psychology at the University of Chicago, studies how seasonal changes in day length and temperature cause changes in animals, including their circadian and ultradian rhythms. In a recent study published in PLoS ONE, he and Irving Zucker from the University of California Berkeley looked at how changing day lengths affected the reproductive physiology and activity levels in male Siberian hamsters. By manipulating the light and dark periods these little guys were exposed to, they were able to pinpoint the day length that triggered changes in their biological clocks.
The Siberian hamster, which is often kept as a pet, originated in Siberia, Mongolia and Manchuria, areas where the landscape is vastly different during the winter and summer. Prendergast said this makes them good candidates for studying seasonal changes.
“They live in a world where the energetic landscape is very different in the winter versus the summer,” he said. “In summer there’s plenty of above-ground food, it’s abundant and temperatures are warm. During the winter there’s basically no food around. The animals do still forage all winter long. They don’t hibernate throughout the winter, and this has driven a lot of adaptations in their physiology.”
During the winter, the hamsters grow thicker coats, shut down their reproductive systems, eat less and lose weight to conserve energy. Prendergast and Zucker wanted to see how the shorter days of winter changed their ultradian rhythms as well, so they took hamsters that had been living in captivity with a standard daylight period of 15 hours and moved them to cages with different light and dark periods, between nine and 15 hours. They tracked activity levels to monitor changes in their ultradian and circadian rhythms, and measured body weight, fur color and testes size to track physiological changes.
They found that after 12 weeks, the hamsters exposed to day lengths of less than 12 to 13 hours started to exhibit physical changes: their fur became lighter, body mass decreased and testes size decreased. At that same threshold, the hamsters’ ultradian rhythms also became more robust, with longer periods of activity as the days grew shorter but at wider and more consistent intervals. In the wild, this means that at some point in the fall the shorter day length triggers changes in the hamsters’ bodies and their internal clocks to prepare them for the winter ahead.
This is great strategy to survive winter for these cute little furballs, but how does that relate to humans?
“We have very important ultradian rhythms that guide our behavior, and understanding the nature of the clocks that drive these is a worthwhile venture,” Prendergast said.
For instance, in a separate study, Prendergast and Annaliese Beery from Smith College studied the effects of pregnancy on the ultradian rhythms of hamsters. As soon as the female hamsters became pregnant, they shifted from a pattern of very consistent, well-defined circadian activity to an ultradian pattern of activity and rest. This would more closely mimic the non-circadian activity patterns of her pups, which would allow her to adapt to their feeding schedule.
“Instead of having her physiology driven by a clock which oscillates every 24 hours, she’s now being driven by a clock that oscillates every three to four hours. That may facilitate this on-demand feeding of the pups,” Prendergast said. “This is true of newborn humans as well. Newborn babies don’t have robust circadian rhythms, but feeding by the mother is beautifully ultradian.”
Like all basic science, studying these building blocks provides a foundation for more complex applications in humans. “Testing these things in humans is difficult and expensive,” Prendergast said. “So you want to get a handle on physiological substrates, environmental cues or hormonal cues that can push the clock around in smaller animals. Then you can say, ‘How can we extrapolate this into understanding humans better?’ ”
Prendergast BJ, & Zucker I (2012). Photoperiodic influences on ultradian rhythms of male siberian hamsters. PloS one, 7 (7) PMID: 22848579