Victoria Creo pink eyes, why do they do this?

I think it has something to do with how much light they get. I put mine on rotations between back row and front row, and sometimes after a long shift the ones coming from the back row have dark eyes. examples: http://i.imgur.com/T0YAj.jpg, http://i.imgur.com/jdol5.jpg

I thought it had something to do with that, but I wasn't sure if it helped with their night vision or if it was their version of shutting their eyes.

LOVE the eyes!