The UV rays that tan the skin may also trigger the release of endorphins, suggesting there's a scientific reason why some people seem addicted to tanning. Experiments conducted on mice reveal that after a week of moderate UV exposure--not enough to cause a sunburn--the levels of beta-endorphins in the mice's blood had risen so sharply that they blunted sensations of pain. Mice that were later given an opioid-blocking drug (usually administered during heroin overdoses) went through typical withdrawal symptoms. "Companies seem to have learned from experience that tanning has a propensity to be addictive," a lead author on the study said. "So it's the perfect business model."