Light is energy. When objects are illuminated, energy is being released.
Visible light will fade dyes, ultraviolet causes a chemical change and
infrared heats up the process. Light damage is cumulative and

When this energy is being projected on an object for an
extended duration, a photochemical reaction can occur affecting the
molecules that make up the object. For example, many paper products are produced with high levels of lignin (providing strength) that is very reactive and susceptible to photochemical deterioration. When exposed to light, the lignin causes the paper to yellow and become very brittle. Generally works on paper and textiles are the most vulnerable objects to the affects of light due to the delicate nature of paper, and the fibers and dyes used in textiles.

Due to the dangers of light, museum make very specific decisions regarding how objects should be illuminated—types of lights, strength of illumination and duration of display.  The Museum may choose very low light levels and may use a schedule for rotating objects on display.  In the home, collectors should avoid putting art and artifacts in direct contact with windows with strong natural light and may also considering rotating what they have on display.

An experiment to see the damage that light can inflict on a paper artifact:

Take a newspaper, fold in half and put one side in the direct sun for a few hours.  Now look at the paper, notice color changes and fold and tear to see the difference in the strength of the paper.