Prolonged exposure to the sun can cause significant fading to your clothing, curtains, and upholstery. Signs of damage occur in similar fashion to the picture. (This is an actual photo of a couch that has been nearby a window for approximately 4 years).
Notice the difference in fabric? Also, we often see this type of damage on golf polos underneath the collar. The protected area remains the original color, but the area expose to sun lightens significantly. After this damage occurs, there is no way to reverse the effects.