Why do some people spend so much time in the sun, despite knowing that excessive exposure puts them at risk for skin cancer?
A new study suggests they are addicted to ultraviolet light, whether from the sun or tanning beds. UV light raises levels of beta-endorphins, so-called "feel good" chemicals in the body.
Beta-endorphins, the body's natural opioids, are stimulated by drugs such as heroin and cocaine. They are also released into the bloodstream by smoking cigarettes or drinking alcohol, encouraging the addictive behavior.
Researchers have found that ultraviolet light may stimulate the same protein pathway, causing a slavish devotion to sunbathing or regular trips to the tanning parlor.
Every day, for six weeks, researchers at Massachusetts General Hospital exposed a group of shaved mice to UV light that was the equivalent of spending 20 to 30 minutes in the midday Florida sun. The dose of ultraviolet light was calculated to induce skin tanning but not sunburn.
Dermatologist David Fisher, director of the Cutaneous Biology Research Center at Massachusetts General in Boston, led the study, which found that blood levels of beta-endorphin rose significantly in the exposed mice.
Fisher says the beta-endorphin molecule acts like an anesthetic so that mice exposed to ultraviolet did not respond to light touch or heat, compared to control animals. When the exposed mice were given a drug that blocked the natural opioid pathway, they became agitated and began shaking and squeaking through chattering teeth.
Addicted to light
It appeared that the mice, like drug addicts, had become hooked on UV light.
Fisher said the UV exposure resulted "in behavioral changes, in addiction, in withdrawal symptoms. And this suggests that ultraviolet radiation can have significant opiate-like effects in a broad sense – probably in many species, not only in laboratory mice."
The exposure "perhaps may underlie some of the dangerous consequences of UV radiation in man," Fisher added.
Sunlight is a primary source of vitamin D, a nutrient essential for skeletal formation and bone strength. Fisher said it's possible the addictive nature of sunbathing is a throwback to prehistoric times when sunlight was the only source of vitamin D and periods of daylight were short.
Could taking vitamin D supplements, which are cheap and easily available today, treat a sunbathing addiction?
"We don't know," Fisher said. "Could it be that vitamin D itself participates in some of these behavioral effects? That's a very interesting speculation. It happens to be something we are looking at at the same time."
The study suggesting the potential addictiveness of ultraviolet sunlight is published in the journal Cell.