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It’s not often that I wish I could turn into a horse. But sometimes, standing shoulder-to-shoulder on a crowded bus or train home after a tiring day, I do envy them one thing: they can fall asleep on their feet. 

In truth, I'd probably slump over immediately if I actually dozed off while standing. This is because I am human and our muscles relax during sleep. The body isn’t really designed to hold itself upright once that happens. But horses and other equine animals, such as zebras and donkeys, have a special adaptation that allows them to pull it off  — for the most part, anyway: It’s called the “passive stay apparatus,” and it’s a special anatomical feature that lets these animals lock their legs in place while resting.

“‘Passive means that without muscular effort, the leg is prevented from being collapsed or flexed,” explained Victor Cox, an associate professor at the University of Minnesota’s College of Veterinary Medicine.  

The stay apparatus is present in all four limbs, although the way it works differs slightly between the front legs and the hind legs. In both cases, though, it involves a complex series of interactions among joints and tendons in the legs, which allow the animal to lock itself in a standing position with very little muscular activity.

The passive stay apparatus involves a complex series of interactions among joints and tendons in the legs, which allow the animal to lock itself in a standing position with very little muscular activity.

Thanks to this ability, horses are able to spend the majority of their time upright — in fact, according to Cox, most horses spend up to 90 percent of their time on their feet. That said, experts believe that even this adaptation doesn’t allow the animals to go through a complete sleep cycle standing up.

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Research suggests that horses can achieve slow-wave sleep while standing. This is a deep-sleep phase during which the brain becomes less responsive to stimuli and the sleeper is more difficult to wake.

Not all of a horse's nod-off time happens on hooves: Experts believe that horses typically have to lie down — and, yes, they do spend a small amount of their time on the ground — in order to achieve REM (rapid eye movement) sleep. One 2008 study in the Journal of Veterinary Internal Medicine, which used electrodes to monitor horses’ sleeping patterns, actually documented a horse achieving REM sleep while upright. Even this animal, however, ended up buckling and slumping forward shortly afterwards.  

It seems to be a fairly common belief that many other animals, such as cows, giraffes and elephants, do all their sleeping standing up. In fact, most of these animals lie down to sleep just like the rest of us — it’s just that many spend a lot less time sleeping than humans do and are rarely caught on the ground. Giraffes, for instance, do sleep lying down, with their long necks bent over their backs. But some limited research suggests that they sleep in short bursts at a time, and generally not for more than four or five hours in total throughout the day.  

Experts believe that horses typically have to lie down — and, yes, they do spend a small amount of their time on the ground — in order to achieve rapid eye movement, or REM, sleep.

And, of course, there’s the old urban legend about “cow-tipping,” the idea that it’s possible to creep up on a cow as it sleeps standing upright and push it over. In actuality, the idea of cow-tipping has been largely dismissed as a total myth — primarily because it would require inhuman strength to actually push a cow over, but also because cows typically sleep lying down. In fact, according to Cox, cattle may spend up to 50 percent of their days lying down, whether they’re sleeping or not.  

The reason for the short-sleeping schedules in many of these animals probably has to do with safety — the less time spent on the ground, the less likely it is that a predator will sneak up and eat you. And the leg-locking adaptation found in equines is generally believed to serve the same purpose, helping horses stay on their feet, ready to flee at a moment’s notice, even when dozing, for as much of their time as possible.  

When it comes to us humans, though, no such adaptation has evolved. So while we may be doomed to spend our daily commutes bleary-eyed but awake, we can at least look forward to a long (and horizontal) rest at the end of the day.