long standing exposure to the suns UVA and UVB rays can result in Sun damage (photodamage) of the skin. To protect itself from the damaging effects of the sun, your skin increases its production of the dark brown pigment called melanin. Additionally, collagen and elastin begin to break down, causing the skin to lose resilience resulting in fine lines, wrinkles and sagging. Daily exposure to UV light then accelerates this aging process and may cause uneven coloring.
Science: Your skin increased the production of melanin in order to protect itself fromt he damaging effects of the sun. The melanin commonly makes your skin look darker or tanned. Sometimes there are uneven levels of melanin produced, causing irregular pigmentation (colouring) of the skin.