Can hair naturally change colour?

[HighlandWorrier]

It's a weird one, DH has dark brown hair and I've just noticed one side is distinctly lighter than the other. He has never dyed his hair nor has he changed shampoo. Initial thought was sun exposure but he says why would it only affect one side. He reckons it's stress as work is not a happy place right now. Has this happened to anyone else?

Is it the side that the sun hits while he is driving? I regularly see people with one arm tanned more than the other because of this and the sun does lighten my hair so....

Going grey happens like that, it's patchy at first, sounds normal to me.