Given the numerous warnings linking sun exposure with skin cancer, why do so many people still worship the sunshine?

According to new research published in the journal Cell, it may be because our bodies become addicted to the feel-good hormones that are released through exposure to ultraviolet (UV) radiation. This craving may guide individuals to actively seek out UV rays, which are well known to increase a person’s risk for skin cancer and melanoma.  According to the American Cancer Society (ACS), 76,100 new cases of melanoma will be diagnosed in 2014.

“The linkage between all common forms of skin cancer and UV is very firmly established and has been known for many years,” senior study author Dr. David E. Fisher, chairman of the department of dermatology at Massachusetts General Hospital (MGH) and Harvard Medical School, told “On the one hand, we know a huge amount of what causes [skin cancer], and you’d expect the incidence to be falling. In fact, epidemiological data shows not only that it’s rising, but rising with a steeper slope than any other cancer.”

It has been suggested that heavy outdoor or indoor tanning habits qualify under psychological criteria for addiction-like behavior. Researchers speculated that exposure to UV radiation triggers the release of hormones called endorphins, which help relieve pain by activating opioid receptors – the same receptors activated by prescription painkillers, morphine and heroin.

For their study, Fisher and his team exposed shaved mice to UV light for six weeks in low doses. After one week, measured endorphin levels in the bloodstream increased. The subjects were then tested for common symptoms associated with opioid signaling – such as low sensitivity to touch and temperature – and the researchers found the mice had become numb to sensory input. When the mice were put on an opiate-blocking drug, to lessen the effects of the increased endorphins, the numbness was instantly reversed.

“It was the first clue that UV radiation was affecting real opiate signaling to the level of being able to perceive or act on a sensory input,” Fisher, who is also director of the melanoma program at the MGH Cancer Center, said.

In another experiment, when UV-exposed mice were put on an opioid-blocking drug, they had withdrawal symptoms including shaking, tremors and teeth chattering, suggesting chronic UV exposure leads to physical dependence and addiction-like behavior.

Study authors equivocated chronic exposure to UV radiation in their study to approximately 20 to 30 minutes of ambient midday sun exposure in Florida during the summer, for a fair-skinned person of average tanning ability.

To further study the UV exposure and endorphin connection, researchers wanted to know whether the subjects’ behavior could be influenced by cognition of withdrawal symptoms.

“In a human population, if you wake up on a gorgeous sunny morning, you have the choice of going to the beach and getting heavy UV exposure or saying you’re not going to do that because you know it’s bad for you; we have a choice,” Fisher said. “We wondered when mice were exposed to this type of effect, does it reach a point where it actually motivates them to behave in a fashion we could actually measure.”

After being given opioid-blocking drugs, the UV-exposed mice actively avoided conditions under which they suffered withdrawal symptoms, the researchers found. For the general population, this finding may mean a different approach is needed to convince people to avoid hazardous UV exposure.

“From a public health perspective, it’s important because it suggests, to break the UV exposure pattern – which is probably partially responsible for the skin cancer incidence we see – maybe it’s more than telling you, ‘The sun is bad for you and associated with skin cancer,’” Fisher said. “Maybe we need to be imparting the message on a deeper level, toward, ‘You may be actually opiate-addicted to some degree.’”

One solution may be to better legislate access to indoor tanning beds, especially for teens.

“To the extent these are currently being regulated as cosmetic devices with wide open access, when we’re dealing with something that is opiate stimulating, that’s a place we should be dealing with more strongly,” Fisher said.

As for the belief that our bodies “need” sun exposure to produce essential vitamin D, Fisher noted that the “argument really doesn’t hold water anymore.” While vitamin D is vital for bone health and calcium regulation, getting it from the sun isn’t a safe method, he said. Plus, a wide range of variables such as time of day, location, clothing, season of the year, and skin pigmentation, make sunlight an unreliable source.

“You’re essentially taking vitamin D with a carcinogen,” he said. “Even more than that, as a doctor, if a person’s vitamin D level is too low, it’s not even a reliable way to maintain vitamin D level.”