The short days of winter may serve as a cue for the immune system to be ready for action.

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.

Moreover, they raise questions about the role of light changes in human immune function.

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

There are hints 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 is mediated by hormones called glucocorticoids, which include cortisol.

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

Proceedings of the National Academy of Sciences March 19, 2002;99:4067-4072