Sun tanning or simply tanning is the process whereby skin colour is darkened or tanned. It is most often a result of exposure to ultraviolet (UV) radiation from sunlight or from artificial sources, such as a tanning lamp found in indoor tanning beds. People who deliberately tan their skin by exposure to the sun engage in a passive recreational activity of sun bathing. Some people use chemical products which can produce a tanning effect without exposure to ultraviolet radiation, known as sunless tanning. Moderate exposure to the sun contributes to the production of melanin and vitamin D by the body, but excessive exposure to ultraviolet rays has negative health effects, including sunburn and increased risk of skin cancer, as well as depressed immune system function and accelerated aging of the skin. Some people tan or sunburn more easily than others. This may be the result of different skin types and natural skin colour, and these may be a result of genetics. Several cases of tanning addiction have been reported by medical researchers. 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 underlie the addiction.