Why does human skin tan after exposure to sunlight?

Human skin tans as a natural defense mechanism against damage from ultraviolet radiation found in sunlight.1

When the skin is exposed to UV rays, melanocytes—specialized cells in the epidermis—increase their activity to produce more melanin. It is a common misconception that the number of melanocytes increases during this process; in reality, the existing melanocytes simply become more active, transferring more pigment granules to neighboring keratinocytes.23

Once inside these skin cells, the melanin granules often position themselves above the cell nucleus, forming a protective shroud that absorbs UV radiation and limits its ability to penetrate and damage deeper tissues. This increased pigmentation is the body's attempt to shield itself from further injury. The tan persists as long as these pigmented cells remain in the upper layers of the skin; once they die and are shed, the tan fades.13