UV Tan

Just like when you tan under the sun, when your skin is exposed to UV light, cells in your skin called melanocytes are stimulated to produce skin pigments called melanin. The melanin then surfaces on your skin, coming into contact with oxygen, where it starts to turn brown.
UV light from the sun hits your skin [from sun or lamps] and stimulates special cells called melanocytes.
Melanocytes start producing skin pigments called melanin.
The melanin travels to the surface of your skin where it comes in contact with oxygen.
It "oxidizes" causing the melanin to turn brown. Your skin has developed a tan.

Mystic Tan

With a spray tan, the process is different. The DHA (dihydroxyacetone) in the spray reacts with the amino acids in your skin and this stimulates a chain of reactions that cause the cells in your upper epidermis to turn brown.
A sunless spray tan (Mystic Tan) does not involve the production of melanin like a UV tan. The tanning spray solution contains the active ingredient DHA (Dihydroxyacetone), a sugar molecule that creates a tanned appearance in the upper layer of the skin (epidermis).
This stimulates a chain of reactions that causes the cells in the upper epidermis to turn brown. This process takes about 2 to 4 hours. The intensity of the color depends on the level of DHA and the amount of amino acids (proteins) in the skin.
A sunless spray tan lasts about 5-7 days, the average life span of the outer epidermis.