Why does my leopard gecko keep squinting?

My Leo, Charlie, has been squinting lately. I don’t think it’s because he does not like the light because he squints in the dark too. He has been doing it for about a week and a half now and I’m worried something is wrong with him. I’ve heard that Leos squint because of a bad shed, but I thought that he might have gotten sand in his eye. Why is he squinting and is there a home remedy for it?