Everyone knows about the winter blues - but now scientists have found evidence that we might be healthier during the summer.
It has been acknowledged for some time that various diseases are more common depending on the time of year. But the University of Cambridge study published today (Wednesday) is the first to show that this may be down to seasonal changes in our immune system. Words: PA
It helps explain why we are more likely to suffer certain conditions - including mental illness, diabetes, arthritis and heart disease - in winter months while people tend to be healthier in summer.
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.
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."
The islands of Kiribati in the Pacific Ocean are some of the sunniest islands in the world. They straddle the Equator and are located close to the patch of the ocean that absorbs more rays than any other piece of water on Earth. Lie on the white sand beaches, try your hand at world-class fishing and visit the numerous WWII ruins, all while soaking up the sunshine, of course!
With over 300 sunny days per year, Andalucia in Spain is one of Europe's sunniest destinations and a centre for 'sun and sand' tourism. The region sees 12 hours of sunshine a day in July, its sunniest month. Ecija in Seville is known as 'The Frying Pan of Andalucia' as it records the highest summer temperatures in Spain. In the winter, Seville, Andalucia's capital, experiences just seven to eight days of rain too, leaving plenty of clear weather and around six hours of sunshine a day.
Looking to hit the ski slopes AND top up your tan? "One of the best places for sunny skiing in Europe is on the Sierra Nevada in southern Spain," says Leon Brown at The Weather Channel. "The resort is 3,400 metres high and often above the clouds! On a clear day you can see the Atlas Mountains to the south in Morocco." With the mountain range located in Andalusia, you can enjoy a high number of sunny days in the winter and ski under blue skies with warm temperatures in the morning, before heading for the beach and sunbathing in the afternoon. It's no wonder Sierra Nevada has been dubbed the world's sunniest ski resort.
With its vast desert landscapes, it's no surprise that the Middle East is a seriously sunny spot on Earth. The Weather Channel's Leon Brown says: "Globally the sunniest places and resorts all year are Dubai, Muscat and resorts in North East Oman, Abu Dhabi, Bahrain and Kuwait with 13 to 14 hours of sunshine in the summer and 10 hours in the winter." Abu Dhabi (pictured) sees 3,609 hours of sunshine a year.
Not only is Las Vegas one of America's sunniest spots, with the sun shining 85 per cent of daylight hours, it also has an average of 210 clear days in the year and sees little cloud, with only Phoenix and Yuma, both in Arizona, enjoying more clear days. Las Vegas has an annual average of 3,825 sunshine hours and when the sun finally goes down, there's the famous nightlife to enjoy and countless casinos to visit.
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With up to 10 hours of sunshine a day and 3,863 hours a year, Aswan in Egypt is one of the planet's sunniest places. July is when you'll see the most sunshine in Aswan and there's 375 hours of it to enjoy. Set on the East Bank of the Nile, you can stroll along the Corniche, eat freshly-caught fish in one of the floating restaurants and visit ancient temples.
Alice Springs in Australia’s Northern Territory experiences an average of 146 sunny days a year, as well as 10 hours of sunshine a day during summer. The best part is that the Australian summer is our winter so you can enjoy temperatures of up to 36C in January while your friends at home are wearing their woollies! Leon Brown at The Weather Channel says Alice Springs sees up to 14 hours of sunshine in the summer months and up to 10 in the winter.
Ok, so we know the UK is nowhere near one of the world's sunniest places but we know you're wondering which part of the British Isles does see more sunshine than others. The Weather Channel's Leon Brown tells us the "sunniest places tend to be the south coast around the Isle of Wight to Eastbourne with 16 hours of sunshine in mid-summer." Not bad, eh? Leon adds: "Sometimes the Channels Islands are sunnier, plus in the spring the Western Isles of Scotland can be the sunniest."
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