NEW YORK (Reuters Health) - The short days of winter may serve as a cue for the immune system to be ready for action, new research in hamsters suggests.

Scientists found that hamsters kept under "short-day" conditions that simulated the light patterns of winter had a more rapid immune response to stress compared with hamsters who basked in summer-like light.

The findings suggest that waning daylight periods cue the animals' immune systems to prepare for plunging temperatures, scarcity of food and other stressors of winter, researchers report in the March 19th issue of the Proceedings of the National Academy of Sciences.

Moreover, they raise questions about the role of light changes in human immune function, the study's lead author, Staci D. Bilbo of Ohio State University in Columbus, told Reuters Health.

It is well-known that certain illnesses, such as the common cold, follow a seasonal pattern. But, she noted, the effect of season and daylight on immune function is unclear.

There are "hints," Bilbo said, that latitude is related to the incidence of disease in humans, but the potential role of day length in immune response and disease has not been studied much.

In her team's experiments, hamsters living in short-day conditions maintained higher blood levels of various immune-system cells than their summer-living brethren did. The short-day animals also had greater concentrations of the stress hormone cortisol.

What's more, under acute stress, immune cells in the short-day animals were quicker in manning their posts in the skin and other key points of defense against injury and infection. Such "trafficking" of immune cells, the researchers note, is mediated by hormones called glucocorticoids, which include cortisol.

This does not, of course, mean that these animals are less likely to be injured or die in the winter. Bilbo pointed out that in the real case of winter, animals must contend with a greater number of threats, such as more extreme weather and lack of food. Instead, the study suggests that light cues help animals' immune systems prepare for winter conditions.

And a problem in translating such findings to humans is that, even in winter, most people live in relative comfort with heat and adequate food.

"Humans aren't really in their natural habitat," Bilbo pointed out.

Still, she said further study of the relationship among daylight, hormones and immune function in humans would be worthwhile. Already, Bilbo noted, research in the petri dish has shown that the hormone melatonin, which is mainly released at night, can trigger human immune cells to proliferate.

SOURCE: Proceedings of the National Academy of Sciences 2002;99:4067-

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