Did You Know?
Horses and cows can sleep standing up, but they don‘t experience full REM sleep unless they lie 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.
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.