Working on your tan may actually be an addictive behavior—if shaved mice tell us anything. A new study published in Cell followed mice with shaved backs who were given the equivalent of half an hour of bright sun daily for six weeks, the BBC reports. In addition to producing a protein that broke down into the pigment melanin (that's what creates your tan), Massachusetts General Hospital and Harvard researchers found the UV exposure produced something else: pleasure-giving endorphins, which Reuters points out "function through the same biological pathway" as heroin. Mice who received drugs to halt the endorphins' work showed signs of withdrawal: teeth chattering and shaking among them. In a manner typical of addiction, they avoided the area where those drugs were provided.
Discussing addiction, a researcher involved tells the BBC he feels "reasonable confidence that this pathway really does exist and is probably present in everybody," and suggests there's an evolutionary component, with a developed "addiction" potentially related to our need for vitamin D. Other experts, however, aren't so sure. If people were really addicted to UV rays, "you would have people giving up their family lives to get access to sunshine; you would have people who lose their jobs because they spend their day on the beach," says one. And the researchers didn't find that the mice aggressively sought out UV light, nor did they test whether the mice had a preference for UV light or non-UV light. Still, experts recommend sunscreen—perhaps even when you're getting your nails done.