What animal can sleep while flying through the air?

Explanation

Common swifts spend most of their lives on the wing — some stay airborne for close to ten months without landing. Biologists tracking them found the birds nap in short bursts at dusk and dawn while gliding in slow, high-altitude circles, resting one half of the brain at a time while the other stays alert enough to steer and watch for danger. That trick, called unihemispheric sleep, is also seen in some marine mammals, but the swift is the clearest known case of a bird doing it mid-flight.