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As the days become longer and hotter in the march toward summer, many of us feel healthier, livelier. It’s not just a perception: Humans are actually less ill during the summer than they are during the winter.

In the colder months, chronic diseases such as multiple sclerosis, rheumatoid arthritis, and neuropsychiatric disorders flare up; more people die of cardiovascular disease; and more are diagnosed with immune-related disorders such as type I diabetes.

Now, researchers may have discovered why that may be. Almost a quarter of our genes, including those that regulate the immune system, shift with the seasons, according to a first-of-its-kind study published last week in the journal Nature Communications.

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That shift appears to include the heightening of inflammation pathways during the winter, says John Todd, lead author of the study and head of the JDRF/Wellcome Trust Diabetes and Inflammation Laboratory at the University of Cambridge in the United Kingdom.

“It’s good to have inflammation in the winter, because we have more infections. But if that inflammation is sustained or exuberant, you can have tissue damage” he says. “It might tip you over the edge to have a cardiovascular event, or make your multiple sclerosis or arthritis more severe.”

For the study, Todd and colleagues scoured data on the activity levels of 22,822 genes in blood and fat cells of 16,000 adults and children during winter and summer months, starting with data sets from the United Kingdom and then expanding their search around the world, in both northern and southern hemispheres, over eight months of intensive research.

Roughly 10 percent of human genes showed increased activity in the summer; a separate 12 percent were more active in the winter. That shift in genetic activity turned out to be largely because of changes in the composition of blood, says Todd: The types and quantities of the cells that make up our blood changed from winter to summer and back again.

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Notably, a set of genes associated with vaccine response was more active in winter, suggesting that vaccinations might be more effective during those months. Clinical trials will be needed to confirm that hypothesis, says Todd.

More immediately, the results suggest that we might want to try to avoid or reduce inflammation during winter months. That can be achieved by “common sense” efforts, says Todd, such as maintaining a healthy diet, exercising, taking vitamin D supplements, and even traveling to sunny places during winter months. As if we needed another excuse to hit the beach.

MEGAN SCUDELLARI