As someone who has become Gackt like with eyes and sunglasses...Im going to venture a guess that he has astigmatism and the light with contacts is super bright. I just started wearing them again, and I have astigmatism, and man...I definitely need sunglasses.

Remember, Gackt doesn't have much light (if any) in his home, and I can relate to why again. Having to squint because the light is so bright causes headaches, and when I have my sunglasses on, I dont have to squint (so much.)