When they wake up, they have lost half of their body weight. But by then, it's warm outside, and time to go bug-hunting again. Truthfully, bats can see fine. But since most bats come out at night or sundown, they don't rely on their eyes very much. In the dark, a bat's excellent hearing is much more important.
Bats keep their big ears very clean.
While they are flying, they make high-pitch squeaks. Some of these squeaks are too high for people to hear. But the bats hear them. When those squeaks hit a tree or a bug, the sound bounces back at the bat. As soon as he hears it, the bat knows what is in front of him.