Why do bats sleep upside down?

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The primary reason is that bat wings are much weaker than birds, which means they can’t lift their body vertically like sparrows. As a result, bats need to enter the gliding position first then flap their wings to generate thrust, in much the same way of airplanes. If they want to take off from the ground, they need to run like swans swan take off, which is very clumsy. While taking off from above is rather straight forward, because they can enter the gliding position directly with the help of gravity, which enabled them to take off instantly like sparrows (very ingenious, isn’t it?). BTW, eliminating the need of running also simplified their legs, which saved much weight.

Hanging upside down is much easier than standing, because hanging is a mechanically stable position. While standing, their legs need to maintain their balance all the time, so their legs can’t be too weak. This again simplified their legs, and saved the weight. Another thing is that hanging is less energy consuming than standing. Although sounds not much, it should be noted that bats are the only flying animals that have hibernation, so saving energy is always more favorable.

Although hanging upside down confers many advantages, they are not without cost. Here are several cons and the ways by which bats overcome them.

Landing. Landing is not trivial for flying animals, but landing upside down is even more tricky, especially landing on the ceiling instead of landing on branches, where there is a space constraint. Yet bats came up with their own solution. Bats are unique among all flying animals as their wings are supported by four fingers plus two legs, which have 25 movable joints to control the wing shape. Such design made bats the most agile flyers ever (even taking into account the extinct pterosaurs), which enabled them to land upside down with ease, even for the largest flying foxes How Do Bats Land Upside Down? | National Geographic & Falling with Style: Bats Perform Complex Aerial Rotations by Adjusting Wing Inertia

Taking a dump. Although being a little shy to talk about, pooping is a serious problem for animals that rest upside down, because it’s not funny to have those mess flowing over your body. A surprising fact is that bats are not as messy as we thought. Bats have thumbs extending out of their wings, which act as hooks to allow them to hang upright for a while, until their business is done, without worrying messing on their fur. Biology - All About Bats

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Size. Although hanging saved a lot of trouble, it also put a constraint on their size, because getting too large will make it difficult to find a place to land. While OTOH, ground landing flyers don’t have such problem at all. As a result, the largest flying fox spans 1.8 meters, and weights 1.6 kg. For comparison, the largest pterosaur spans 12 meters, and weights over 200 kg; the largest swan spans 3 meters, and weights 13 kg.

So although hanging has a lot of advantages, it also comes with costs. However, the unique structure of bats makes them ideally suitable for this lifestyle, which should explain why bats are the only flyers choosing this way.