Horses have a unique "stay apparatus" in their legs that lets them lock their joints and sleep standing up without falling over.
These tall giants often take quick naps standing up to stay alert for predators.
Elephants sleep standing up, especially when in the wild, to be ready to flee from danger at a moment's notice.
Flamingos often sleep while balancing on one leg. This helps them conserve body heat and energy.
Zebras sleep standing up to quickly escape predators. They usually sleep in herds, so one zebra is always alert.
Cows often take short naps standing up, especially when they're out in the pasture. They do lie down for deeper sleep, though.
Buffaloes, like cows, can doze off on their feet but will lie down for longer rest periods
Deer are light sleepers and often nap while standing to stay vigilant against predators.
Sheep can sleep standing up but prefer to lie down in comfortable and safe environments.
Penguins can sleep standing up to keep warm in cold climates, often huddling together in groups.