The drop and fly theory
When you watch a bird take off from the ground, you will notice they need a run-up. In order to get off the ground, flying animals needs to achieve what scientists call “lift” to overcome gravity. Many big birds, and bats, do not have strong enough wing muscles to generate the lift required to take off from a standing position (like a helicopter can).
Bats cannot run so it would be almost impossible for them to take off from the ground. A major advantage to hanging upside down is that bats do not need to generate lift to begin flight. They just drop out of their bed, open their wings and off they go.
Did you know there are seven species of bats that do not sleep upside down? They sleep in curled-up leaves! Six of these species live in Central and South America, while the other one lives only in Madagascar.
- source: https://www.latrobe.edu.au/news/articles/2018/opinion/why-do-bats-sleep-upside-down
