Repeated visits to the beach or tanning salon could be signs of an addiction, according to a study in mice.

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David Fisher at the Massachusetts General Hospital in Boston and his team found that mice chronically exposed to ultraviolet light produced an opioid called β-endorphin, which numbs pain and is associated with addiction. The mice also showed signs of withdrawal, including shaky paws and chattering teeth, after treatment with an opioid-blocking drug. The effects of β-endorphin, however, were not as pronounced as those seen in earlier studies of mice given morphine.

If the results hold true in humans, they could explain why many tanners (pictured) continue to seek out the sun, despite the known risks.

Cell 157, 1527–1534 (2014)