Isn't it a Little Dark?

I ride the subway everyday to get to work and to go home. I always notice that some people wear their sunglasses inside, and I wonder why. The subway is underground, and doesn't have the greatest lighting. I know that there are times when the subway does see the light of day, but for the most part it's underground.

This is in the same boat as people wearing their sunglasses at night. WHY?

If anyone has the answer as to why people wear sunglasses in dark places, please enlighten me.

Thanks,

J.