why do bats sleep upside down
http://www.whyzz.com/why-do-bats-sleep-upside-down
it is easier for them to cover thier face hanging upside down so the light doesnt' shine in thier eyes.
They're blind, light doesn't affect them. They see using sonar. They screech at the area around them and when the sound echos back to them, they can tell what's in the area by the change in the sound.