It’s almost impossible for you to fall asleep while standing. The moment you began to doze, you’re likely to fall down. But horses often seem to sleep while they’re standing up, and they do!
When a horse stands perfectly still and relaxes, joints in his legs lock into position, holding the horse up even if he falls asleep.
Horses actually prefer to sleep standing up, as when they lie down, their weight presses them down heavily against the ground, which can make breathing difficult. So a horse rarely lies down to sleep, and can doze quite comfortably while standing in his stall.
Certain other big animals, like the elephant, can sleep while they’re standing up too!