A suntan is the skin’s natural form of sun protection. It is created by releasing the melanin from the melanocytes when the skin is exposed to UV radiation. When the melanin moves to the surface, it absorbs UV radiation.
The time it takes for melanin to rise to the skin’s surface after production has been stimulated takes around 12 hours. A suntan is not a high degree of sun protection as it is only what could be rated as about an SPF3 sunscreen.
When the skin is overexposed to UV radiation, there is not enough time for the body to react, resulting in sunburn.
By following safe sun protection practices, a suntan can be built up and extend the period one can stay in the sun by an SPF3. However, if the resulting skin's UV index changes, the natural SPF will not help much.