A Norwegian red cow sleeps in Lillehammer, Norway. Like most big mammals, cows can doze off on their feet but sleep deeply lying down.
For starters, the idea that these animals only sleep on their feet isn't right. Most four-legged land herbivores—cows, moose, rhinos, bison, and horses among them—can doze lightly on their feet, but they have to lie down to sleep deeply.
For instance, "when horses appear to be sleeping standing up, they can either be in a state of drowsiness or what is known as slow-wave sleep, which is not quite as deep as REM [rapid-eye movement] sleep," Amy Johnson, of the University of Pennsylvania School of Veterinary Medicine, says via email.
A Norwegian red cow sleeps in Lillehammer, Norway. Like most big mammals, cows can doze off on their feet but sleep deeply lying down. [Photograph by Vincet J. Musi, National Geographic Creative]
Even still, for those of us who can't sleep in a crowded airplane, sleeping standing up sounds impossible. So how does it work?
"The legs of horses [and other animals] have what's known as the 'stay apparatus,'" Johnson explains. Their limbs contain tendons and ligaments that allow the animal to remain standing with minimal muscular effort, and thus allow them to stand—and even doze—for long periods.
Herd animals such as bison have another reason for standing a lot: To keep an eye out for danger.
In a bison herd, "not every animal is asleep—or lying down for that matter—at the same time," Murray Woodbury, of the Western College of Veterinary Medicine in Saskatchewan, says by email.
"This allows the awake and/or standing herd members to act as sentinels against predators or other danger."
Surprisingly, some big animals need little shut-eye.
Giraffes can function on as little as five—yes, five—minutes of slumber a day, according to the San Diego Zoo. Both African and Asian elephants don't sleep much more than three hours.
Rhinoceroses are a little more like us: They sleep about eight hours a day, according to Kruger National Park in South Africa.
A man lies among orphaned baby rhinos at the Ol Pejeta Conservancy in Kenya. [Photograph by Ami Vitale, National Geographic Creative]
Vince Crichton, a retired wildlife biologist at Manitoba Conservation and Water Stewardship in Canada, has had firsthand experience with snoozing moose—he once fell asleep beside one.
Crichton had been observing a wild bull's behavior for two days, and when the animal laid down in the forest, Crichton gingerly sat down nearby, "talking moose with him."
After a few hours, Crichton says he was "rudely awakened" by his wife, who asked, "Are both of you going to sleep all afternoon?"
Man and moose had both nodded off ... but neither of them on their feet.