When the sun is shining, many of us are unable to resist a trip to the beach to soak up the rays, despite recommendations that we should cover up to reduce the risk of skin cancer. And now, researchers have discovered why; ultraviolet radiation from the sun releases endorphins - "feel-good" hormones - that act like a drug, making exposure to sunlight addictive.

The research team, including senior author David Fisher of the Massachusetts General Hospital and Harvard Medical School in Boston, MA, recently published their findings in the journal Cell.

Skin cancer is the most common form of cancer in the US. According to the Skin Cancer Foundation, over 3.5 million skin cancers are diagnosed in more than 2 million people every year.

Approximately 90% of nonmelanoma skin cancers and 86% of melanoma skin cancers - the deadliest form of the disease - are caused by exposure to ultraviolet (UV) radiation from the sun and tanning beds or lamps.

To reduce the risk of skin cancer, public health experts recommend that people stay in the shade when the sun is at its strongest (usually around midday), wear clothing that covers arms and legs, wear a wide-brimmed hat and sunglasses, use sunscreen with a minimum sun protective factor (SPF) of 15 and avoid indoor tanning.

However, many of us tend to ignore these recommendations due to a desire to feel the sun on our skin.

According to Fisher and his colleagues, previous studies have indicated that many people who adopt UV-seeking behavior meet the clinical criteria for substance addiction. But Fisher notes that the mechanisms underlying UV addiction have been unclear.

UV exposure 'caused addiction-like behavior and physical dependence' in mice

In an attempt to unravel the workings behind UV addiction, the team conducted a study in which shaved mice were exposed to UV rays 5 days a week for 6 weeks.

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Love to tan? Researchers say that UV radiation releases endorphins - "feel-good" hormones - that make us addicted to sun exposure.

The UV rays the mice were exposed to were the equivalent of a fair-skinned human of average tanning ability being exposed to 20-30 minutes of ambient midday sun in Florida during summer.

After 1 week, the researchers found that endorphin levels in the bloodstream of the mice increased. Endorphins are hormones that relieve pain by activating opioid receptors via the same pathway used by prescription painkillers, heroin and morphine.

After 6 weeks of UV exposure, some of the mice received an opioid-blocking drug. This caused them to experience withdrawal symptoms, such as shaking, teeth chattering and tremors. These mice then avoided box locations where they had received the drug, indicating that excessive UV exposure triggers addiction-like behavior and physical dependence.

Commenting on the team's findings, Fisher says:

"This information might serve as a valuable means of educating people to curb excessive sun exposure in order to limit skin cancer risk as well as accelerated skin aging that occurs with repeated sun exposure.

Our findings suggest that the decision to protect our skin or the skin of our children may require more of a conscious effort rather than a passive preference."

Fisher says it is "surprising that we're genetically programmed to become addicted to something as dangerous as UV radiation." He hypothesizes that it could be down to vitamin D synthesis - the body's main source of vitamin D is the sun.

"However," he adds, "in the current time, there are much safer and more reliable sources of vitamin D that do not come with carcinogenic risk, so there is real health value in avoiding sunlight as a source of vitamin D."

Medical News Today recently reported on a study published in the journal Nature, which suggests that using sunscreen alone is not enough to protect against melanoma skin cancer.