We all need a vacation every once in a while, and we usually try to time it so we can escape the seasons we dread. And apparently, some of the genes responsible for keeping us healthy also like to squeeze in some seasonal relaxation.
Scientists in the UK and Germany have shown that nearly a quarter of our genes become either more or less active depending on seasonal changes, which means we could be more prone to illnesses at different times of year.
The team says their results help to explain why we're generally healthier in the warm summer months, and might also provide some clues into a range of diseases that are known to have a higher incident rate in winter.
"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," senior author and geneticist John Todd, from the University of Cambridge, told Dani Cooper at ABC Science.
Certain tropical diseases, and other conditions like type-1 diabetes, multiple sclerosis, and cardiovascular disease, have known seasonal patterns. The researchers in this study wanted to test whether there might be some kind of physiological explanation for this.
Because inflammation is a risk factor for many of these diseases, the team was particularly interested in whether the genes that promoted or suppressed inflammation changed their activity depending on the season.
They analysed white blood cells, fat tissue samples, and the expression of individual genes from more than 16,000 people living in the northern and southern hemispheres, including diabetics from the UK, and people with asthma from the US, the UK, Australia and Iceland. The team also examined a cohort from the Republic of the Gambia in West Africa to gain some insight into people living near the equator.
The study, published in Nature Communications, found that 5,136 out of 22,822 genes tested - roughly 23 percent - changed their expression level depending on the season when the sample was taken. A gene is 'expressed' when it is active in a cell or tissue, and generating proteins.
"Remarkably, seasonal genes displayed opposing patterns in the southern and northern hemispheres," the researchers wrote.
In European populations, inflammation was more common during the winter months of December, January and February. In Australia, it was more common during the months of June, July and August. And genes that suppressed inflammation were more active during the respective summer months of each hemisphere.
In the Gambian populations, the researchers found that seasonal gene expression and immunity peaks during the rainy season, from June to October. This could be an adaptation to help protect against diseases like malaria, which are more common during this time of year.
In people from Iceland, fewer seasonal genes and patterns were identified. "If a seasonal photoperiodic clock exists in humans, the impact of living at higher latitudes requires further exploration," the researchers wrote.
The team say their data could be used to better time vaccine administrations, and also say the work suggests that distinct environmental adaptations in immune activity have occurred independently in different populations.
Merlin Thomas from Baker IDI Heart and Diabetes Institute in Australia, who wasn't involved in the study, told Cooper at ABC Science that seasonal variations in immunity are very common in nature.
"We might have air-conditioning in summer and central heating in winter, but it appears we still change with the seasons like most other animals on the planet," he says.
"Whether these variations can explain why some illnesses are more common at some times of the year than others is unclear. All illnesses are multifactorial. The time of the year may be one small component in the puzzle."