Ever notice how healthy you feel in the summer, while conditions such as cardiovascular disease, diabetes, and rheumatoid arthritis tend to flare up around the winter months? This isn’t the result of more time spent at the beach, but rather seasonal changes in how our immune systems function. A recent study conducted at the University of Cambridge has revealed that the activation of around a quarter of our genes associated with the human immune system is influenced by the time of the year.

"This is a really surprising — and serendipitous — discovery as it relates to how we identify and characterize the effects of the susceptibility genes for type 1 diabetes," Professor John Todd, director of the JDRF/Wellcome Trust Diabetes and Inflammation Laboratory, said in statement. "In some ways, it's obvious — it helps explain why so many diseases, from heart disease to mental illness, are much worse in the winter months — but no one had appreciated the extent to which this actually occurred.”

Todd and his colleagues gathered data taken from more than 16,000 people living on both the northern and southern hemispheres, like the United States, United Kingdom, Iceland, Australia, and the Gambia. Samples taken from each participant included blood and adipose tissue (fat). Researchers examined the cell types found in each blood sample and measured the level of “gene expression,” or when it is active in a certain cell or tissue.

Out of 22,822 genes studied by the research team, 5,136 were expressed differently depending on what time of the year blood and adipose tissue samples were taken during. Although changes in seasonal genes varied in northern and southern hemispheres, these differences were present across geographical and ethnic backgrounds. Due to nearly 24 hours of daylight during the summer and nearly 24 hours of darkness in the winter, evidence of seasonal activity was not as strong in residents from Iceland.

“The implications for how we treat disease like type 1 diabetes, and even how we plan our research studies, could be profound,” Todd added. “Given that our immune systems appear to put us at greater risk of disease related to excessive inflammation in colder, darker months, and given the benefits we already understand from vitamin D, it is perhaps understandable that people want to head off for some 'winter sun' to improve their health and well-being."

Since our internal body clock, also known as our circadian rhythm, is affected by changes in daylight, researchers speculated that the seasonal variation of the immune system is caused by environmental cues, such as daylight and temperature. This could explain why shift workers (people who work outside of the 9 to 5 shift) suffer from various health risks.

"This is an excellent study which provides real evidence supporting the popular belief that we tend to be healthier in the summer," said Professor Mike Turner, head of infection and immunobiology at the Wellcome Trust. "Seasonal variation to this extent is a fascinating find — the activity of many of our genes, as well as the composition of our blood and fat tissue, varies depending on the seasons. Although we are still unclear of the mechanism that governs this variation, one possible outcome is that treatment for certain diseases could be more effective if tailored to the seasons."

Among the genes studied by the research team, ARNTL was of particular interest due to its suppression of inflammation, the body’s response to infection. ARNTL is more active in the summer and less active in the winter, meaning levels of inflammation in the northern hemisphere would be higher during winter. The research team will look to focus on a set of genes linked to a person’s response to vaccination to see if vaccination programs are more effective during the winter.

Source: Bonifacio E, Wallace C, Todd J, et al. Widespread seasonal gene expression reveals annual differences in human immunity and physiology. Nature Communications. 2015.