How come the sun bleaches your hair

[DaftyLass]

But tans your skin?
I am confused....

I hadn't thought about it like that but would really like to know.

Why as in the mechanism, or why as in the evolutionary reason?

Because that's different.

Both I guess, I understand why our skin darkens to protect from burns, but why then is the hair getting lighter?

Also thought, it would be nice to know why we don't just have a "somewhere in the middle colour" that doesn't burn or tan?

Basically, what good does it do me to have hair that changes seasonally? It's kind of strange because rabbits and such go pale in the winter to blend into the snow, but people's hair goes paler in summer?

Put very simply:
A tan is a protective measure of a living skin cell.
Hair like nails is dead and bleaches (like clothes put out into the sun)

Also thought, it would be nice to know why we don't just have a "somewhere in the middle colour" that doesn't burn or tan?

Because how evolution works is whatever gives an advantage for either reproduction or survival 'wins'. Beige wouldn't 'win'. When I go to Kenya, I burn to a crisp and would be horribly maladapted. A very dark-skinned Kenyan comes to the North of Scotland and ends up with a vitamin deficiency. Because my pale skin is great for making vitamin D in shady climes, but terrible for the sun.

Hair is mostly about protecting your head. So Afro hair is good for sun, skinny blond hair is recessive so it really makes no difference. Weirdly there is a theory that because hair tends to get darker over time, there is a slight preference for blonde hair because it makes women look younger and therefore more fertile. Hence why it doesn't die out.

The other thing is that evolution isn't conscious. So yes, pale, sparkly skin a la Twilight probably would be best everywhere. That's not how genes work. In Africa, we all had darker skin because that was the most successful mutation. As we migrated, dark skin was still the best in sunny places but pale was better in the North. But we get the mutation we get. Evolution is unconscious.

The sun fades your hair, technically I suppose?

it doesn’t do you any good or any harm if your hair is bleached by the sun. Unlike rabbits you don’t need your hair for camouflage and it’s not even really the thing that keeps you warm. There is no reason for you to need a protective mechanism to stop your hair going light in the sun, hence why one has not evolved. The comparison to suntans and rabbits isn’t really helpful!

As pp said the reason we don’t have one “middle of the road colour” is to do with the need to balance pigmentation which protects from the very real damage sun does to our skin (the radiation damages your dna) with the need to be able to stimulate vitamin d production. I live in the north of Scotland and am very fair. The predominant weather conditions where I live mean I rarely burn unless it is unusually sunny or I go on holiday. We humans have learned to travel and move around in ways evolution hadn’t accounted for so we are no longer perfectly adapted to our individual environments (Australian aborigines have the right skin for the climate they live in, white Australians not so much)

I appreciate the information, thank you!
I hadn't thought about the fact that the hair is dead, and the points about the need to get vit D/better sun protection make sense.