A suntan is a darkening of the skin in reaction to exposure to the ultraviolet (UV) rays in sunlight. The skin pigment melanin becomes darker as UV rays damage the skin.

Although skin gets darker to protect its deepest layers against harm from UV rays, a tan is evidence of some degree of sun damage. This damage is cumulative, which means the more time a person spends in the sun over many years, the more long-term damage is done to the skin.

UV rays from both the sun and indoor tanning also can cause skin damage and increase the risk of skin cancer.