Washington  - Spending time basking in the sun’s warming rays may have addictive properties, claim researchers in the US. Their study, published in the journal Cell, showed that sunlight led to the production of endorphins in the skin. The Harvard Medical School team said their experiments on mice showed repeated UV exposure led to addiction in the animals. Other scientists said describing it as ‘addiction’ was a step too far.  There have been previous suggestions that people can get addicted to the sun. A study of university students using tanning salons suggested around one in three met the definition for addiction. 
Other trials showed withdrawal symptoms could appear, including nausea, jitteriness and shaking, when enthusiastic tanners were given a drug used to treat addiction.
Researchers at Massachusetts General Hospital and Harvard Medical School were exploring how such addiction might occur. Mice with a shaved back were exposed to the equivalent of half an hour of midday Florida sun every day for six weeks. They showed UV radiation led to the production of a protein in the skin called proopiomelanocortin. This is broken down into the pigment melanin, which gives you a tan, but the team showed pleasure chemicals, or endorphins, were also produced.
The chemicals act on the same systems in the body as other opioids such as heroin and morphine. Giving the mice drugs to block opioids, which are used in rehab clinics, led to withdrawal symptoms including shaking and tremors. The mice then started to avoid the place where the drugs were being administered, which the researchers said was a hallmark of addictive behaviour. But their experiments do not show the mice actively seeking out the UV light in the same manner as a drug addict.
Dr David Fisher, one of the researchers from Massachusetts General Hospital, told the BBC: ‘I would say at this stage, with reasonable confidence, that this pathway really does exist and is probably present in everybody. ‘People who may have no intention of using any drugs may just think they’re going out to enjoy a great day outdoors and may be becoming addicted and exposing themselves and their children to UV in a fashion which could elevate their risk of developing skin cancer. ‘It sounds like a cruel joke to be addicted the most ubiquitous carcinogen in the world, it is plausible that there is an evolutionary advantage and we suspect that may relate to vitamin D.’ However, other scientists are not convinced.