Sun tanning or simply tanning is the process whereby skin color is darkened or tanned. The process is most often a result of exposure to ultraviolet (UV) radiation from the sun or from artificial sources, such as a tanning bed, but can also be a result of windburn or reflected light. People who deliberately tan their skin by exposure to the sun engage in sun bathing, though there are also artificial tanning methods. Some people use chemical products which can produce a tanning result without exposure to ultraviolet radiation. Casual exposure to the sun has moderate beneficial impact, including the production of vitamin D by the body, but excessive exposure to ultraviolet rays has detrimental health effects, including possible sunburn and even skin cancer, as well as depressed immune system function and increased risk of accelerated aging. To avoid sunburn, most people apply suitable sunscreen to skin exposed to the sun, but others use oils to accelerate the tanning process.
Some people tan or sunburn more easily than others. This may be the result of different skin types and natural skin color, and these may be as a result of genetics.
Several cases of tanning addiction have been reported. Though the mechanism by which tanning addiction occurs is unknown, some evidence indicates that the release of endorphins during the tanning process causes the pleasurable effects that underly the addiction. Because the pleasurable feelings derived from tanning are caused by endorphin release, they are similar to those experienced during a runner's high.