The primary vitamin synthesized when the skin is exposed to sunlight is vitamin D. Ultraviolet (UV) rays trigger the production of vitamin D in the skin, which is essential for calcium absorption and maintaining healthy bones. This vitamin also plays a role in immune function and has been linked to various health benefits, including mood regulation and overall well-being. Insufficient sunlight exposure may lead to vitamin D deficiency, necessitating dietary sources or supplements to ensure optimal health levels, especially in areas with limited sunlight.