It’s no wonder those winter months make you want to roll over and hibernate when the alarm goes off on cold, dark mornings.

Scientists now know that our urge to burrow back under the blankets is associated with our body’s seasonal clock.

Once daylight hours begin to dwindle after the sunny days of summer, our bodies actually need more sleep. Unfortunately, most of us don’t get enough, a problem that’s been exacerbated by the invention of the light bulb, says an Ohio State University psychologist. Turning on lights at night to stay up later only adds to the sleep debt we’ve already accumulated.

When the debt piling higher, it doesn’t take a doctor or an economist to tell you a crash is coming.

Meanwhile, the wane in daylight also appears to render our immune system more vulnerable. Consider the flu and cold season: During the fall and winter, people are more likely to contract a viral or bacterial infection. For others, autumn triggers a condition known as seasonal affective disorder, a severe depression that spontaneously resolves in the spring.

Researchers at Ohio State have been analyzing the effects seasonal changes have on the health and sexuality of mice and humans.

One such study – published recently in the Royal Society of London’s Proceedings: Biological Sciences – scrutinized deer mice.

Researchers found that mice exposed to eight hours of light per day ended up with immune function stronger than those who were exposed to sixteen hours of light.

As for humans, more study is needed. But rest is always best if your body is telling you it’s tired.