The animal known for its ability to sleep standing up is the horse. Horses are capable of locking their leg joints, which allows them to maintain balance and rest without collapsing. While they can sleep standing up, they also require periods of deep sleep, which they obtain when lying down. Horses have a unique sleep pattern that combines short periods of light sleep while standing with deeper, REM sleep when lying down. This ability to sleep while standing is an evolutionary adaptation that helps them avoid predators in the wild.