There are two types of skin tanning

Short-lasting tan (IPD / immediate pigment darkening): It is caused by a redistribution of melanin, which is already in the skin and is located mainly by UVA rays triggered.
Delayed tanning (DT / delayed tanning): It is caused by UVA and UVB radiation-induced and is based on an increased formation of melanin. The delayed skin browning reached its maximum after five to seven days.
By UV light, but also causes skin damage. In the absorption of radiation in the skin, energy is released. It exerts there a detrimental effect on the connective tissue and DNA (genetic material) from which the nuclei of skin cells are contained. 
The harmful effects of UV radiation is most directly, partly it is also about free radicals conveys, in the superficial layers of skin under radiation exposure from oxygen are formed.