Bats hang upside down to roost and sleep because the bones in their hind legs are lightweight and cannot support a bat's body weight in an upright stance. While they rest, bats wrap their wings around themselves like a cloak.
According to the San Diego Zoo, bats are the only true mammals that can actually fly. Most bats take off flying by dropping while hanging upside down. Many bats cannot take off while on the ground.
Bats normally land by slowing down until they are stalling, then they grab hold of a branch or some other surface. Some bats will perform a flip and then grasp onto a branch.
Bats usually gather in large social groups and rest in caves, man-made structures or trees.