Tanning is commonly caused by ultra violet rays of the sun during incidental, prolonged and unprotected sun exposure. The process of tanning leads to a darkening of the complexion due to an increase in the melanin content of the melanocytes. Both UVA and UVB rays are responsible for tanning. The UVB rays are more harmful and can lead to skin cancers. Certain medications increase the sensitivity of the skin to the harmful effects of the sun, causing uneven dark patches.