Human skin tans as a protective response to exposure to ultraviolet (UV) radiation, primarily from sunlight. The tanning process involves several biological mechanisms:

1. Melanin Production: When your skin is exposed to UV light, it triggers cells called melanocytes to produce more melanin, the pigment responsible for giving color to your skin, hair, and eyes. Melanin acts as a natural sunscreen, absorbing, scattering, and reflecting UV radiation to protect the deeper layers of the skin from damage.
2. DNA Protection: Prolonged exposure to UV rays can damage DNA in skin cells, potentially leading to mutations that can cause skin cancer. Melanin helps to absorb UV radiation before it can reach and harm the cell’s nucleus, where genetic material is stored.
3. Suntan Types: There are two types of suntans – immediate (also called immediate pigment darkening) and delayed (a lasting tan). Immediate darkening occurs within minutes of exposure and fades quickly; it’s caused by the movement of existing melanin toward the surface of the skin. Delayed tanning happens over hours to days and results from increased melanin synthesis.
4. Fair vs. Dark Skin: People with fair skin typically produce less melanin and are more susceptible to sunburn and skin damage. Those with darker skin naturally have more melanin, which provides better protection against UV radiation but does not mean they are immune to sun damage.

It’s important to note that although tanning is a protective mechanism, excessive exposure to UV radiation can overwhelm this defense, leading to sunburn, premature aging, and an increased risk of skin cancer. Therefore, using sunscreen and practicing safe sun exposure habits are crucial for maintaining skin health.