Why do bats sleep outside down?

Question

Yes, bats sleep upside down. But why?

Think of a human, sleeping upside down. I'll fall down in 20 seconds if I was you.

Answer 1

I think I remember seeing something on a documentary that said their legs are not strong enough to support their body weight.

Answer 2

It's a behavior that they adapted largely because the roof of a cave is a very safe place to sleep...not much can reach them there.