Do you like to spend your days basking on the beach or relaxing in a tanning bed? You may think you do it for cosmetic reasons—that natural glow does look good on you—but new research suggests you might have another motive. Mice frequently exposed to ultraviolet (UV) light show symptoms of drug use and addiction, suggesting that every time you seek out the sun’s rays, you may just be looking for a high.

“This is an idea that has been staring us in the face forever,” says Steven Feldman, a dermatologist at Wake Forest University in Winston-Salem, North Carolina, who has studied the psychological effects of tanning. “There’s been a sudden rise in skin cancers, and dermatologists have been cautioning patients about sun exposure,” he says. “Yet people aren’t cutting back on their time outdoors, and the whole industry of tanning beds has grown extraordinarily fast.”

Skin cancer is indeed on the rise in the United States. Between 1992 and 2006, the number of treatment procedures for slow-growing nonmelanoma skin cancers increased by almost 77%. Society’s obsession with good looks and beach-bronze skin may be one factor, but prior research has shown that something else could be going on. Frequent tanners experience withdrawal symptoms when given a drug that overrides the effects of opioids, chemicals that can be naturally produced by the body and generate feelings of euphoria. Tanners can also tell the difference between beds emitting UV radiation and non-UV light, saying that they prefer UV beds, which make them feel more relaxed.

Those factors led David Fisher, a physician and skin researcher at Massachusetts General Hospital and Harvard Medical School in Boston, to wonder, “Could there be something deeper that’s driving people—despite their knowledge and intent to be safe—to put themselves in harm’s way?”

To find out, he and his colleagues gave mice a daily dose of UV light for 6 weeks; it was equivalent to what a light-skinned person would receive while lounging in the midday Florida sun for 20 to 30 minutes. The team then measured levels of β-endorphin, an opioid produced by the brain that’s responsible for feelings of pleasure, and analyzed the rodents’ tail positions and sensitivity to pain and temperature. The idea is that opioids cause animals to become less aware of discomfort. For instance, if they’re being poked with a pin or are sitting on a warm plate, they won’t quickly notice. Their tails, which are normally floppy, can also straighten out if they’re on opioid drugs, like morphine.

The team’s findings, published online today in Cell, reveal significant elevations in endorphin levels—ranging from 30% to 50%—after mice got their daily hit of rays. The animals also demonstrated decreased touch and temperature sensitivity and walked with rigid, erect tails—all signs that the endorphins were producing opioidlike effects. On the other hand, control mice with no exposure to UV exhibited normal behavior, and a genetically engineered group that was unable to produce β-endorphins remained unaffected in the presence of UV rays.

To verify that UV light was causing these effects, the researchers injected the mice with naloxone, “the drug that you give a person in the emergency room who has a heroin or opioid overdose,” Fisher says. It disrupts the endorphin process and blocks the reception of opioids, preventing the body from getting high. The researchers found that after a dose of the drug, the animals’ sensitivity and tail posture immediately went back to normal, and they underwent withdrawal symptoms, such as body shakes, chattering teeth, and paw tremors. Later, they even demonstrated a preference for cages where they had not experienced naloxone’s miserable effects, suggesting that the desire to avoid UV withdrawal can affect behavior and choices—a sign of addiction.

“I thought it was quite elegant, a very nicely done study,” says Bryon Adinoff, a psychiatrist at the Veterans Affairs North Texas Health Care Center and the University of Texas Southwestern Medical Center in Dallas who studies drug and alcohol abuse. “But I don’t believe what they demonstrate says anything about addiction,” he says. “Addiction is when you start doctor shopping to get more, going on the streets to find some, and stealing to support your habit. They did not approach it from the perspective of how much will an animal seek out UV light.”

Still, Adinoff thinks the study shows a pretty decisive link between UV radiation’s effect on the skin and how it spurs the production of chemicals that induce feelings of reward in humans and animals.

The research team speculates that the rise in endorphins and the druglike effects evolved as a way to encourage mammals to seek out the sun, which spurs the body’s production of vitamin D. But with tanners possibly becoming addicted to the high—spending too much time in salons and outside—the risk of cancer increases, and the proclivity can become deadly.

“I have nothing good to say about tanning salons,” Fisher says. “I think they’re addictive, they elevate your risks for skin cancer, and now there’s the concept that they cause a behavioral dependency—it’s the nicotine story all over again.”