This damage results when energetic photons collide with molecules in the fiber of the sail material. In some of these collisions, the energy and angle of contact is just right to break a molecular bond. While each of these breaks is insignificant by itself, the cumulative effect is not. Over time, the damage mounts until the fibers lose much of their cumulative resistance to flex, stretch, and breakage. Gradually, they become more likely to succumb to the other stresses in the life of a sail.
Different molecules have different sensitivity to deterioration from light.
Plant fibers like cotton and linen apparently evolved with good resistance to light deterioration. Artificial fibers, which were developed for other properties, in some cases have much less tolerance to light. Polyester, Kevlar and Vectran, while possessing progressively greater modulus and tenacity, also are progressively more intolerant to light. Perhaps the extreme example of this trend is PBO, a fiber with incredible stretch resistance and flex tolerance, but one that proved so quickly crippled by light exposure as to be of little practical use as a sail material. When selecting fabrics for sails, sensitivity to light exposure is only one of several criteria to use.