Researchers have known for some time that various diseases are more common depending on the time of year. But a new study by University of Cambridge is the first to show that this may be down to seasonal changes in our immune system.
Professor Mike Turner, head of infection and immunobiology at the Wellcome Trust, said: "This is an excellent study which provides real evidence supporting the popular belief that we tend to be healthier in the summer.
"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."
Researchers studied samples from people living in both the northern and southern hemispheres, including the UK, USA, Iceland, Australia and The Gambia.
They found that the activity of almost a quarter of 23,000 genes tested differed according to the time of year, with some more active in winter and some in summer.
One particular gene, known as ARNTL, which has been associated with suppressing infections in mice was more active in summer, suggesting it may have a similar effect in humans.
It is not known exactly what causes these variations, but researchers say it may be down to environmental cues like daylight and ambient temperature.
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Our internal body clock - known as our circadian rhythm - is in part coordinated by changes in daylight, which explains why people in jobs that do not fit with the daily cycle, such as factory shift workers or crews on long haul flights, can be affected by poorer health, researchers added.
The differences were found across different nationalities but were less pronounced among Icelandic donors.
This may be due to the near-24 hour daylight during summer and darkness during winter, the study said.
Researcher Chris Wallace said: "We know that humans adapt to changing environments.
"Our paper suggests that human immune systems adapt to show different seasonal variation in equatorial regions with fewer distinct seasons compared to regions at higher and lower latitudes with more pronounced differences between winter and season."
Professor John Todd, director of the JDRF/Wellcome Trust Diabetes and Inflammation Laboratory, said: "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.
"The implications for how we treat disease like type 1 diabetes, and even how we plan our research studies, could be profound.
"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."