Unlike birds, bats are unable to begin flight from the ground as their wings are not powerful enough and their legs lack the necessary strength for a running takeoff. They use their front claws to climb to a high spot and launch themselves off to achieve flight. By sleeping upside down, they are able to quickly and easily begin flight.
Another reason they hang upside down is that bat’s talons work opposite to most muscles. When they relax, their talons shut, meaning they do not need to exert any energy if they hang upside down. Contracting the muscle releases them and allows flight to begin. In addition, bats can roost in places that predators or other animals might not be able to find.