How Does Sun Bleaching Work?
The ultraviolet rays [2] of the sun help break down the melanin pigment of your hair. Breaking down this pigment helps to make your hair lighter. Because hair is made of dead tissues, any reduction in the level of melanin is almost permanent and your hair becomes lighter.
Changes can be seen after staying a few hours in the sun especially in people who initially have a light hair color. People with very dark hair may notice no changes despite staying in the sun for a very long period.