Nearly a quarter of our genes change how active they are according to the season, which may explain why people are more prone to illness in winter rather than summer, scientists say.
In a study published on Tuesday in the journal Nature Communications, the researchers said they were stunned by just how much of our genetic code is affected by the time of year.
Out of 22,822 genes the researchers analysed, 5136 operated at higher or lower levels depending on the season, with some more active in winter and others more active in summer.
Our immune system could be one of the many mechanisms that would be subtly affected by "seasonality", the researchers said.
Lead scientist John Todd, a professor of medical genetics at the University of Cambridge, said the discovery was both "really surprising" but also "obvious".
"It helps explain why so many diseases, from heart disease to mental illness, are much worse in the winter months," he said.
"But no one had appreciated the extent to which this actually occurred.
"The implications for how we treat diseases like Type 1 diabetes, and how we plan our research studies could be profound."
The team looked at blood samples and fatty tissue provided by 16,000 people living in the northern and southern hemisphere and in equatorial Africa.
The activity - in scientific parlance, the "expression" - of thousands of genes in the fatty tissue varied according to the time of year the samples were taken.
There were also seasonal differences in the types of blood cells.
The "seasonal genes" displayed opposing levels of expression in the samples taken from the northern and southern hemisphere.
The difference was most marked among donors from Iceland, where there is nearly 24 hours of daylight in summer and nearly 24 hours of darkness in winter.
There were also variations, but far less pronounced, from samples taken from people in equatorial regions, where the seasons are less distinct.
In samples from the west African country Gambia, genes in immune cells in the blood were more active during the rainy season from June to October, when mosquito-borne diseases such as malaria are on the rise.
One discovery is that a gene called ARNTL that plays a part in inflammation - the defensive process that is also implicated in many auto-immune diseases - is seasonally influenced.
Another is that vaccination may be more effective in winter than in summer, because a key set of "seasonal genes" in the immune system are primed to deliver a stronger response.
The work will boost understanding of how light and ambient temperature can affect physical and mental health, the researchers hope.
The circadian, or body, clock has been highlighted by a slew of studies in recent years showing the toll of night shifts and jetlag on factory workers and long-haul flight crews.