The Ohio State University study found that changes in the length of daylight in autumn trigger enhancements of the mice's immune system in preparation for the challenges of winter.
These enhancements are most likely the result of increased levels of the hormone melatonin, which is responding to lower levels of daylight in autumn.
However, the study showed that the immune enhancements are not indefinite -- within 32 weeks, the immune system returns to its non-enhanced status, even if short days remain.
These results suggest that melatonin does not work indefinitely to trigger enhancements in the immune system in mice, and similar results may hold true for humans, said Randy Nelson, co-author of the study and professor of psychology and neuroscience at Ohio State.
"Melatonin may lose effectiveness over time if it used regularly, based on what we found in this study," Nelson said.
Brian Prendergast, a postdoctoral fellow in psychology at Ohio State and Nelson published their findings in the November 8 issue of the journal Proceedings of the Royal Society of London: Biological Sciences.
Prendergast and Nelson conducted the study with deer mice. Using artificial light, they had some mice live with long days (16 hours of light per day) for 32 weeks. Another group of mice had short days (8 hours of light per day) for 32 weeks. A third group had long days for 20 weeks followed by short days for 12 weeks.
The mice were tested for immune function throughout the study.
Results showed that the animals exposed to short days for only 12 weeks scored higher on several measures of immune function -- such as the number of splenocytes (a type of immune cell)-- compared to mice exposed to short days for 32 weeks, as well as compared to mice exposed only to long days.
"The immune system is enhanced for a period of time during short days, but we found that after about 20 weeks of short days the immune system spontaneously begins to return to its previous, non-enhanced state," Nelson said. By 32 weeks, the change is complete.
Why does the immune system lose its enhanced status after a period of time under short days?
Nelson said the reason may be related to another seasonal change that occurs in mice during winter - the shutdown of the reproductive system.
This study, and others previously, found that male mice show regression of gonads and other reproductive tissues during exposure to short days. The energy reserves devoted to reproduction are probably redirected to the immune system to help mice survive the winter, he said.
However, the mice cannot wait until the long days of spring for the reproduction system to become functional again. "It takes about 8 weeks for the animal to go from a completely regressed gonad to fully functional sperm production," Nelson said. "If mice waited for spring before their reproductive system began redeveloping, it would be summer before they would be ready to reproduce."
That's why the reproductive system spontaneously begins operating again before winter is over - and before the days begin to lengthen - in order to make sure reproduction is possible when spring begins. That may help explain why the enhanced immune system has to end before winter does, Prendergast said.
Although it has not yet been proven, it is likely that the changing amount of daylight acts through melatonin to produce the changes in the immune and reproductive systems of the mice, Nelson explained. If melatonin acts similarly in humans, it may mean people should be cautious in using melatonin supplements to self-treat problems like sleep disorders.
"After a certain period of time, melatonin may stop working," Prendergast said.
The study was supported by grants from the National Institutes of Health and the National Science Foundation.
Additional contact: Brian Prendergast, (614) 538-9540 brianp@psy.ohio-state.edu
Written by Jeff Grabmeier, (614) 292-8457; grabmeier.1@osu.edu