Sun tanning describes a darkening of the skin (especially of fair-skinned individuals) in a natural physiological response stimulated by exposure to ultraviolet radiation from sunshine or from artificial sources such as a tanning bed. With excess exposure to ultraviolet, a sunburn can develop while Sunless tanning is UV-free.

Two different mechanisms contribute to the UV-induced darkening of the skin. Firstly the UVA-radiation generates oxidative stress which in turn oxidises pre-existing melanin. This leads to rapid darkening of already existing melanin. The second mechanism is the increased production of melanin (melanogenesis). It is a reaction of the body to photodamage from UVB. This melanogenesis is triggered by the same DNA damage that causes sunburn. Melanogenesis leads to delayed tanning. It first becomes visible about 72 hours after exposure.[1] The tan that is created by an increased melanogenesis lasts much longer than the one that is caused by oxidation of existing melanin.

Darkening of the skin is caused by an increased release of the pigment melanin into the skin's cells after exposure to ultraviolet radiation. Melanin is produced by cells called melanocytes and protects the body from direct and indirect DNA damage absorbing an excess of solar radiation, which can be harmful. Depending on genetics, some people can darken quickly and deeply whereas others do not darken much at all.

The ultraviolet frequencies responsible for tanning are often divided into the UVA (315 to 400nm wavelength) and UVB (280 to 315nm wavelength) ranges.

The skin produces vitamin D in response to sun exposure (in particular, UVB waves in the 285nm to 287nm range), which can be a health benefit for those with vitamin D deficiency. In 2002, Dr. William B. Grant published an article claiming that 23,800 premature deaths occur in the US annually from cancer due to insufficient UVB exposures (apparently via vitamin D deficiency). This is higher than 8,800 deaths occurred from melanoma or squamous cell carcinoma. This does not mean that sun tanning is categorically safe or beneficial. Spending several minutes in the sun is long enough to obtain your daily dose of vitamin D. Another research estimates that 50,000,3,000 individuals in the United States and 19,000 - 25,000 in the UK die prematurely from cancer annually due to insufficient vitamin D.

Another effect of vitamin D deficiency is osteomalacia, which can result in bone pain, difficulty in weight bearing and sometimes fractures. This work has been updated in Grant et al. 2005[7] and Grant and Garland, 2006 In addition, it was reported that in Spain, risk of non-melanoma skin cancer is balanced by reduced risk of 16 types of cancer [Grant, 2006]

According to research conducted in 2007 by Cozen, Gauderman, Islam, and Mack , sun exposure during childhood prevents multiple sclerosis later in life.

Ultraviolet radiation has other medical applications, in the treatment of skin conditions such as psoriasis and vitiligo. Sunshine is informally used as a short term way to treat or hide acne, but research shows that in the long term, acne worsens with sunlight exposure and safer treatments now exist (see phototherapy).

satin ribbonsSome people choose to sun tan without clothes to maximize tanning coverage, maximizing health benefits of sun exposure, increasing the sensitory experience and reducing or eliminating tan lines caused by the contrast of exposed and unexposed tanned skin. While some are content to simply sun tan in the privacy of their own private property, some governmental agencies have responded to more demand for clothing-optional sun tanning in public spaces.

As an example, Englischer Garten in urban Munich has meadows where nude sun tanning is the norm. In Denmark, clothing-optional sun tanning is the default dress code on all beaches, with the exception of two. In other areas of the world, clothing-free sun tanning could be met with citation or fines. Clothing-optional beaches (also known as naturist, nude or nudist beaches) and other areas where quiet use or traditional use is tolerated, generally allow for such opportunities without fear of legal harassment or penalty. Even more beaches allow topfree tanning for women. Some describe beaches where nudity or topfree tanning is tolerated to be "more European". Geographic areas in the US with warm to tropical climates with extensive beach fronts often cater to such tourist opportunities, e.g. Haulover Beach, Gunnison Beach, Black's Beach and Baker Beach.