Re: Why does sunlight cause human hair to change color?

I did not know the answer to your question so asked one of my colleagues, 
Dr. Clifford Hall III. His answer is below. 


I do not know all the details but I think that it has something to do with
when the hair grows and color is deposited into the hair.  When the
keratins are originally forming, they are soft and at the same time the
melanins which give hair color is deposited into / onto the keratin
structure. In order for this to occur, the keratins must be flexible or
soft. Once the keratins become hard they no longer can bind the melanins.  

The answer, then, is that when the hair is exposed to sunlight, the UV rays
cause the keratins to harden more quickly - thus less melanin can bind to
the newly formed keratins (i.e. hair).  The lightening of the hair color is
due to the formation of new hair which has a lighter color blending into
original hair color,  giving the hair the appearance of being lighter. 

In the winter, the UV exposure is less and the keratins have the ability to
remain softer longer and thus bind more melanin pigments. This accounts
for a regeneration of hair pigment after the end of summer.

I hope this helps. If you need more information please let me know and I 
will see if some of my other colleagues have more on this topic.