* Sunlight is the key: Hair fading is primarily caused by ultraviolet (UV) radiation from the sun. UV rays break down the pigment molecules (melanin) in the hair, causing it to lighten.
* Darkness blocks UV rays: Darkness means there is a lack of sunlight. If there is no sunlight, there are essentially no UV rays present.
* No UV, no fading: Without UV radiation, the melanin in the hair will not be broken down, and therefore the hair won't fade.
Essentially, the presence of darkness prevents the sunlight from reaching and affecting the hair.