The seasons appear to have a profound effect on how human genes work, according to scientists.
This may explain why some illnesses are aggravated in the winter, they say in Nature Communications.
They found genes involved with immunity - the body's defence against infection - were more active in cold months.
And while this helps fight off viruses such as flu, it may trigger or worsen conditions, such as arthritis, where the body attacks itself, they say.
The international team of researchers analysed blood and tissue samples from more than 16,000 people living around the world.
Of the 22,000 genes they scrutinised - which is nearly all the genes humans possess - a quarter showed clear signs of seasonal variation.
The gene changes that interested the researchers the most were ones involved with immunity and, specifically, inflammation.
During cold, winter months - December to February for people living north of the equator and June to August for those in the southern hemisphere - these genes were more active.
When they studied people living close to the equator, where the temperatures are fairly high all year round, they noticed a different pattern. Immunity and inflammation was linked to the rainy season, when diseases such as malaria are more rife.
In Iceland, where it is cold most of the time, they found fewer seasonal changes.
Prof John Todd, one of the study authors, who is based at Cambridge University in the UK, said the findings could explain why people were prone to certain diseases at particular times of year.
Inflammation plays a significant role in conditions such as rheumatoid arthritis, type-1 diabetes and heart disease, which peak in the winter in countries such as the UK.
"In the UK, we see a rise in new cases of type-1 diabetes in January, February and March, for example," Prof Todd said.
"Our results suggest that part of the reason for this is heightened inflammation and that gene activity is involved."
Prof Todd said it was hard to tease out precisely what was happening, since many factors influenced an individual's chance of developing a disease.
Likewise, diseases and other factors, such as nutrition and stress, could affect how genes function.
Tim Spector, professor of genetic epidemiology at King's College London, said: "Another dimension that could be as important are our gut microbes, which also change between seasons and could be driving these changes because of seasonal changes in diet."
Prof Tim Hubbard, also from King's, said there might be an evolutionary advantage behind the seasonal changes the researchers found.
And Prof Todd said: "In prehistoric humans, these seasonal changes in inflammation would help fight infection."
Another seasonal change they saw was in genes linked to metabolism.
"Presumably these would help with conserving energy to survive when there is little food and shelter," Prof Todd said.
"In modern society we have warm clothing and heating but we still respond to colder temperatures and shorter days.
"But that increase in inflammation could now be a risk factor for diseases of modern life."
The work was funded by the Wellcome Trust and the Juvenile Diabetes Research Foundation.