Why don't birds fall when they sleep in trees?!
Birds are somewhat magical because they have special legs and feet that grip branches while they sleep, so they don't have to worry about falling. When a bird lands on a branch and bends its leg downwards, a special set of tendons behind the leg which run from the underside of the toes to the muscle behind the top of the tibia (similar to the knee joint in humans) automatically pulls and tightens. As a result, the foot automatically bends and becomes rigid, locking the bird in place, even when sleep prevents conscious control of the muscles.
This is one of the reasons why a small bird, weighing only a few ounces, can cling to a twig or branch even in strong winds and storms.
Amazingly, this technology is being imitated in the development of aerial robotic platforms with landing and object-grabbing capabilities! Glory be to Him who perfected everything He created and began the creation of humankind from clay.