Asleep on His Feet

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Asleep on His Feet

So, the next time you're in your bed worrying about whether your horse is comfortable, know that his anatomy has its own "bed" and most likely that stay apparatus is helping him get all the rest he needs.


After a long day at a horse show, I'm ready for a nice cool drink, a hot bath, and a good night's sleep in my comfortable bed. It's hard to accept that my horse, for all his hard work at the show, gets nothing more than a layer of shavings to bed down in. And while he might stretch out and snooze a bit after our long day, most of the time he's perfectly happy spending his sleep time standing up.

Equids (horses, asses, and zebras) are the only animals that sleep standing up. According to the Department of Natural Sciences at the Florida Museum of Natural History, the ability to sleep on the hoof evolved as a way to remain ever alert for predators. Being prey animals, equids could flee faster if they napped on their feet rather than on the ground--it took much longer to wake up, get up, and run rather than to just wake up and run. That head start meant the difference between staying alive and becoming a tasty meal. There are two other theories as well: The longer an animal could stand, the more grass he could eat, and lying down to snooze was too difficult for his ever-increasing body size.

"Horses can and do lie down," says Hilary Clayton, BVMS, PhD, Dipl. ACVSMR, MRCVS, McPhail Dressage Chair in Equine Sports Medicine at Michigan State University's McPhail Equine Performance Center. "But because of their weight, they usually sleep upright on their chests. Sometimes horses do sleep flat out on their sides, but usually for about 15 or 20 minutes. Young foals will sleep on their sides longer, but this is because they don't have the same body weight problem."

"Because they are so heavy, horses can't lie down for long periods of time because it will interfere with blood flow," adds Katherine Houpt, VMD, PhD, Dipl. ACVB, emeritus professor of behavioral medicine at the College of Veterinary Medicine at Cornell University in Ithaca, N.Y. "And that's one reason why horses under anesthesia can sometimes have problems. Also, horses don't lie down as much because they don't need as much REM (rapid eye movement) sleep as other species. Horses do need REM sleep, but not a lot."

Now I Lay Me Down to Sleep...

Some people think horses sleep standing up all the time, when in fact they don't. So we must give them a comfortable place with adequate space and bedding to lie down and get back up again. If you have several horses out in the field, they will sleep grouped together with usually one staying upright--awake and watchful while the others sleep.

Although little is known about how sleep deprivation affects performance, researchers do know that if he's unable to lie down for REM sleep, a horse will eventually buckle at the knees and fall down.

"Many horses that are diagnosed with narcolepsy, a condition when an animal or person goes straight into REM sleep and falls down, are actually old horses that find it very painful to get up or down," says Katherine Houpt, VMD, PhD, Dipl. ACVB, emeritus professor of behavioral medicine at the College of Veterinary Medicine at Cornell University in Ithaca, N.Y. "We've found that if they are given enough traction, bedding, and painkillers, they will lie down."

Sharon Biggs Waller

When horses sleep standing up, they are in slow-wave sleep or SWS (also known as sleep of the mind). "The electrical activity in the brain is large and slow if the animal is in slow-wave sleep, so the mind isn't very active," explains Houpt. "When a horse is awake or in rapid eye movement sleep (also known as sleep of the body), the electrical activity waves are short and fast. This means that there are a lot more neurons at work. This sleep is deeper as far as the body is concerned because all of the muscles are relaxed. This is also the point when dreaming occurs. With horses, you sometimes see them run in their sleep, and even neigh."

Although researchers agree that REM sleep is important for horses, they don't know what happens to horses if they don't get enough of it. "Because not much research has been done in this field, we don't know how much REM sleep horses actually need each night," says Houpt. "It's probably minutes, less than half an hour. The slow wave sleep is certainly hours, about three or four." This sleep is apportioned out in short naps and mostly during the night, between midnight and 4 a.m. "It depends somewhat on how much bedding they have," she says. "Horses often wait until they have fresh bedding and then lie down."

The type of bedding makes a difference, too: Researchers have found that horses lie down and sleep more on straw bedding than on wood shavings.

How it All Works

"Horses can rest standing up because they have modifications in their muscles, ligaments, and tendons that allow mechanical use. Without any active effort or energy, horses can use these structures to hold their joints in place," says Clayton.

To break it down further, horses have evolved three ways to allow standing while sleeping: The stay apparatus, the reciprocal mechanism, and the locking mechanism in the stifle joint (see diagrams of these structures here).

The stay apparatus' job is to brace the entire joint system of the forelegs and the pastern and fetlock joints in the hind legs. These fibrous bands take over the muscles' job of straightening the various joints. They act as drawstrings uniting all the joints together. As Clayton mentioned, their task is passive and no rest is required, unlike muscles, whose tasks are usually active and therefore require rest.

Modern-day horses possess a biceps tendon that travels from the top of the upper foreleg bone and attaches to the shoulder blade. The tendon is dimpled, which means it can fit over a structure called the intertubercular crest, or INT, in the center of the shoulder blade. This structure, along with the tension exerted by the triceps brachii muscle associated with the elbow, prevents flexion and collapse of the forelimb. The suspensory ligament supporting the carpus (knee), and the distal sesamoidean ligaments supporting the pastern, prevent overextension of the leg.

Through studying fossils, researchers were able to pinpoint the time when the stay apparatus evolved. They found that the INT was not present during much of the horse's evolution. Dinohippus (5 million years ago) was the first to show the INT. Two million years later during the Pliocene period, the time of spreading grasslands and development of long-legged grazers, the INT was fully apparent.

The haunches work a little differently. When the horse puts most of his weight on one leg, the locking mechanism (a function of the stifle) and the reciprocal mechanism team up to allow one hind leg to lock into place, allowing the other leg to rest.

"In the horse the patella (knee cap) ligament has three parts, not just one like in humans," explains Clayton. The quadriceps lift the patellar ligaments and hook it onto a big knob on the femur. When the stifle (knee) locks, the reciprocal mechanism, which makes the stifle and the hock move together, causes the hock to lock as well and the entire leg is braced. The other leg can then rest on the tip of the hoof. Although these mechanisms help reduce the amount of energy need for standing, the muscles are still in play, therefore the horse will swap legs every few minutes or so to reduce fatigue.

The horse's neck lowers during sleep and is supported by the suspensory ligament of the neck (nuchal ligament).



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