With summer here and the July 4 holiday upon us, millions of people in America and across the world will head to beaches and pools to soak up the sun. Unfortunately, this commonplace activity can drastically increase a person\u2019s odds of developing skin cancer. Public health officials and researchers know that, despite the known risks for skin cancer, some people can develop a physical addiction to the ultraviolet (UV) light contained in sunlight or artificial light and subsequently increase their dangerous tanning participation. In a study published in June 2014 in the journal Cell, researchers from two U.S. institutions explored the underlying reasons sun tanning can become an addictive pursuit. The UV light in sunlight has a beneficial as well as a harmful role in human health. On the positive side, the human body gets part of its natural supply of a crucially important nutrient, vitamin D, from this light. However, the body reaches its maximum capacity for UV light-generated intake of vitamin D in a matter of minutes, and continuing exposure leaves the skin vulnerable to highly damaging UV radiation. Two basic classes of skin cancer are strongly linked to excessive ultraviolet light exposure: non-melanomas and melanomas. The non-melanoma class includes specific forms of cancer such as basal cell carcinoma and squamous cell carcinoma. Forms of melanoma include superficial spreading melanoma and nodular melanoma. In the U.S., roughly 90 percent of all cases of non-melanoma stem from sun tanning or tanning with an artificial UV light source. Approximately 86 percent of all cases of melanoma stem from the same causes. More Americans have skin cancer than any other form of cancer. An Addiction to Tanning Numerous researchers have demonstrated that repeated exposure to natural or artificial UV light can create the basic conditions for physical dependence, a brain alteration normally associated with recurring overexposure to drugs or alcohol. Dependent individuals have a physically established need to keep participating in the activity that has altered their baseline brain function. People frequently exposed to significant amounts of natural or artificial UV light can also develop a range of symptoms found in physically dependent substance users who develop full-blown addictions to drugs or alcohol. In the context of tanning, these symptoms can include a repeated urge or craving for continued tanning participation, a need to spend increasing amounts of time participating in tanning, the onset of withdrawal when the body\u2019s established need for UV exposure goes unfulfilled and continued tanning involvement in the face of serious or potentially fatal health risks (i.e., skin cancer). Why Can You Get Addicted? In the study published in Cell, researchers from Children\u2019s Hospital Boston and Harvard Medical School-affiliated Massachusetts General Hospital used laboratory experiments on mice to explore the underlying mechanisms that can make tanning participation a physically addictive activity. During the experiments, these mice had their hair shaved off and, over a period of a month and a half, were exposed five times weekly to amounts of UV radiation roughly equivalent to that received by a fair-skinned individual tanning in summertime Florida for half an hour. Specifically, the researchers looked for increases in the mice\u2019s levels of internally generated painkillers called endorphins, which produce their effects by accessing the same brain pathways used by highly addictive opioid drugs (PDF) and medications. Mice were used because the human endorphin system and the mouse endorphin system work in basically the same ways. After just one week of relatively modest UV light exposure, the mice started to experience increases in the levels of endorphins circulating in their systems. At the end of the six-week period of UV exposure, the researchers gave some of the mice naltrexone, an anti-opioid medication that shuts down the brain pathways used by endorphins and opioid drugs and medications. In response to the introduction of naltrexone, these mice developed clear symptoms of withdrawal. In turn, after experiencing withdrawal, the mice typically stayed away from the locations where they had received the medication. The study\u2019s authors concluded that the increased endorphin levels in people who participate in sun tanning or artificial tanning play a critical role in the development of tanning addiction. Essentially, people addicted to tanning experience physical reactions roughly analogous to the reactions of people who get addicted to opioid drugs or medications. The authors believe that their work can provide public health officials with an important tool for explaining the risks of tanning participation.