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How do animals exposed to 24-hour light retain their wake-sleep habits?


The phrase "biological clock" has expanded from scientific observation to American slang. When we hear this phrase, many of us assume it refers to the amount of time left for a woman to start a family. For the scientist, the biological clock refers to a process that took millions of years to evolve – the conditioning of plants and animals by a light cycle that starts with dawn and ends with sunset.

The cycle of dawn and dusk changes with the seasons everywhere in the world (except at the equator, where there is always 12 hours of daylight and 12 hours of darkness). In order to compensate for the seasonal variations of light, mammals likely have an adjustable daily program under the regulation of a biological clock.

But how do mammals in the Arctic – which is characterized by months of full light followed by months of full darkness -- retain their sleep and awake habits in such unusual circumstances? After analyzing the reactions of certain mammals following 82 days of continuous daylight in the summer and 82 days of continuous darkness in the winter, a team of researchers may have begun to identify a clue.

A New Study

The research is captured in a presentation entitled, "Cardiac Physiology of Mammals in Arctic Light Cycle: Heart Rates and Biological Clocks." The authors, G. Edgar Folk, Diana L. Thrift, James B. Martins, and Miriam B. Zimmerman, all from the University of Iowa, Iowa City, IA, will present their findings at the American Physiological Society’s (APS)( annual scientific conference, Experimental Biology 2004, being held April 17-21, 2004, at the Washington, D.C. Convention Center.

Methodology and Results

The researchers analyzed the biological clocks using cardiac physiology. They recorded the mammals’ daily circadian rhythm using heart rate to show the rhythm of sleep and wakefulness.

Control laboratory rats (N=4) were exposed to artificial continuous light and demonstrated the Aschoff effect, where the circadian activity pattern changes quantitatively with the intensity of the light. This group took on a 26-hour day.

This was not the case when the experiment was repeated in the field at the Naval Arctic Research Laboratory - with two species of Arctic rodents exposed to continuous daylight (nocturnal porcupines [Erethizon] N=4, and hibernators, the Arctic ground squirrel [Spermophilis] N=6). Under these circumstances, both species had a specific time of sleep and of wakefulness. In fact, the Arctic rodents, which had undergone 82 days of continuous sun above the horizon, had a crisp, 24-hour day-night rhythm of sleep and wakefulness.


The free-living animals in the Arctic had regular sleep-awake cycles, despite having 82 days of continuous sun. The intriguing question is whether or not these animals have found a clue in the external environment to take the place of the missing sunset. The researchers hypothesize that because the sun during this period is nearer the horizon at one part of the day, this might act as a clue for the biological clocks.

As the American economy requires its work force to abandon traditional work hours of "nine to five," it becomes more important for us to understand how the body’s biological clock can respond to unnatural light clues and adapt to a changing environment. This study is another step in the continuing research towards such comprehension.

The American Physiological Society (APS) was founded in 1887 to foster basic and applied science, much of it relating to human health. The Bethesda, MD-based Society has more than 11,000 members and publishes 3,800 articles in its 14 peer-reviewed journals every year.

Editor’s Note: For further information or to schedule an interview with a member of the research team, please contact Donna Krupa at 703-967-2751 (cell), 703-527-7357 (office) or at Or contact the APS newsroom at 202-249-4009 between 9:00 AM and 6:00 PM EDT April 17-21, 2004.

Donna Krupa | EurekAlert!
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