The answer may surprise you, according to a veterinarian who has investigated how physical or mental discomfort can leave a horse sleep deprived and too tired to stay on his feet.



The 12-year old gelding was standing outside the arena at the state fair waiting for his next class when his head suddenly dropped and his knees buckled. He was almost on the ground before he came to and scrambled to right himself.

His young rider was unhurt but his owners decided to call the veterinarian. Back in the barn area the gelding again nearly collapsed.

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The veterinarian arrived and his initial exam turned up nothing abnormal. He was perplexed. The gelding had no history of illness and had never done this before.

Then a casual conversation with the owners revealed some critical diagnostic information. The horse had been at the fair for eight days, was ridden in several classes daily and fussed over in between. At 10 each evening a large fireworks show was set off directly over the fairgrounds. The sound unnerved the gelding and he remained agitated throughout the night.

Then as if on cue the horse nearly collapsed again. After watching the episode up close the veterinarian asked a seemingly odd question: During the week had the owners seen shavings on the horse that would indicate he had been lying down? The answer was "No."

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After a quick phone consultation with a colleague who had an interest in such cases the veterinarian returned with a diagnosis: The gelding was in simple terms sleep deprived.

For nearly a week the poor horse had not been able to settle into that deep restorative sleep that comes only when he lies down. The veterinarian recommended that the family take the horse home so he could get some much-needed rest. The owners did just that and the gelding never had another episode of near collapse.

The Science of Sleep

As remarkable as sleep deprivation in horses may seem it's not rare. Joe Bertone, DVM, MS, DACVIM, the veterinarian consulted about the gelding at the state fair, has documented nearly 127 similar cases during his 24-year career. These horses he says were unable to get enough paradoxical and rapid eye movement (REM) sleep, the deepest forms of slumber. Bertone recently presented papers on this condition at several national veterinary meetings.

To understand sleep deprivation it's necessary to review the three distinct phases of equine sleep. First comes the deep restfulness phase during which a horse is relaxed but still easily roused. Next comes slow wave sleep: During this phase the horse is even more relaxed but still has some muscle tone and his brain waves charted on an electroencephalogram (EEG) are slow and large. Both of these stages can occur while the horse is standing.

Finally a horse may enter paradoxical sleep so named because the brain is just as active during this phase of slumber as it is during wakefulness. REM sleep, characterized by rapid movement of the eyes under closed eyelids, occurs during this period.

Before entering the paradoxical phase of sleep, "horses will awaken for a moment in the intermediate phase, check out the safety of the environment, and then lie down," says Bertone. "The horse will again enter deep restfulness and then slow wave sleep and then if comfortable in the environment go into paradoxical sleep either lying on its side or tucking its head to the side."

In the paradoxical phase of sleep, rapid eye movements, loss of reflexes and muscle function, and increased brain activity occur. A horse snoozing with one hind leg hitched up may be in the deep restfulness or slow wave sleep phase but to achieve paradoxical sleep the muscles of the body are completely relaxed and he must lie down.

People need about two to three hours of paradoxical sleep daily. Horses in contrast need from 30 to 60 minutes per day, Bertone says. "This information was first published in the '60s and '70s and then presented again in a review paper in the '80s, mostly by French and French-Canadian researchers. But we seem to have forgotten it perhaps because horses need so little of it."

Bertone says that horses don't have daily sleep cycles as people do so they probably don't need to have paradoxical sleep every day.

On the other hand they can go only so long without "lying down for dead-to-the-world slumber," he says. "There is a traditional thought that horses can get all the sleep they need standing up--that's simply not true.

"Based on the cases I've collected and depending on a number of factors the horses that show these clinical signs [of sleep deprivation] can usually go about seven to 14 days without paradoxical sleep but after that we begin to see 'sleep attacks,'" he continues. "However, many horses seem to be able to go far longer."

Although the signs may seem similar sleep deprivation is unrelated to narcolepsy a condition characterized by frequent and uncontrollable periods of deep sleep, says Bertone.

"Narcolepsy is a specific neurological problem in which horses slip nearly instantly into paradoxical sleep and REM without the usual preceding period of slow wave sleep. They collapse to the ground and they don't catch themselves and it's usually in response to some sort of stress, excitement or exercise," Bertone says. "The horses I describe as sleep deprived are simply exhausted. A few days in a comfortable environment and the ability and willingness to get paradoxical sleep are usually the cure."

Physical Causes: When it Hurts to Sleep

Bertone has found that equine sleep deprivation can have a variety of physical causes. Older horses for example may simply be unable to lie down to sleep. That was the problem for a 23-year-old gelding who suddenly began to experience episodes of near collapse several times a day. Fearing the horse was entering his final decline his owners asked Bertone for help. But the physical exam revealed no significant health problems,other than osteoarthritis in both front legs. Bertone asked when the owners had last seen the horse roll. They could not recall. On a hunch that it was simply too painful for the gelding to lie down to rest Bertone prescribed phenylbutazone (bute) for pain control. Sure enough after a few days the old horse was seen lying on the ground sleeping and his collapsing episodes ended.

"In addition to obvious stiffness in joints a huge clue in these cases is a lack of rolling in the history. If they won't lie down to roll they may be too uncomfortable to lie down to sleep," Bertone says. "It isn't unusual for these cases to be given bute or another anti-inflammatory medication for a few days then comfort is reached, the horses lie down, they get paradoxical sleep, and the episodes stop."

Other cases of pain-related sleep deprivation aren't so simple. One horse Bertone encountered was a 12-year-old Quarter Horse gelding who had mysteriously begun to lose weight. In fact the 16-hand animal had dropped from 1,200 to a mere 900 pounds. The physical examination turned up only one significant finding: two small scars and hair loss over his front fetlocks.

Because of his poor condition the gelding was admitted to the hospital for testing. During his stay the staff noticed that the horse would never lie down but occasionally seemed to partially collapse. He would catch himself as his fetlocks buckled which most likely explained the odd scars.

The horse's labwork and physical findings were normal so Bertone decided to investigate further with chest X-rays. "On the way to radiology as I was leading the horse a student noted that there was a strange sound coming from the horse's abdomen. He said it sounded like rocks hitting each other in a river." Bertone asked the student to lead the horse as he listened and he heard it as well. In addition to chest X-rays he requested radiographs of the abdomen. The images revealed two large stonelike mineral formations known as enteroliths in the horse's large colon.

During surgery to remove the stones the surgeon also discovered that the large colon was stuck to the diaphragm. The stones were removed and the diaphragm freed. Almost immediately after the horse returned to his stall after surgery he was flat on his side asleep. "I think every time he tried to lie down the colon pulled on the diaphragm which must have been very painful," says Bertone. "When the colon was freed the poor fellow slept for nearly two days straight." The collapsing episodes never occurred again and the horse gained 200 pounds in three months.

Mental Causes: Too Much Stimulation

Often it's not physical pain but emotional discomfort or stress that keeps a horse from getting adequate rest. A 10-year-old Quarter Horse gelding had been seen several times a day in near collapse when Bertone was called in. The owners were afraid the horse had equine protozoal myeloencephalitis (EPM) but he had no other neurological signs so Bertone knew that this was unlikely. A full physical workup revealed nothing abnormal. The gelding was known to roll frequently which ruled out the possibility that a physical problem prevented him from lying down. Instead Bertone focused on finding a behavioral basis for the gelding's strange problem.

The horse, it turns out, had been purchased and shipped to his new home about two weeks before the first collapsing episodes began. At his previous home he had shared a large pasture with about 150 horses including several mares. In his new pasture the gelding was alone. Bertone suggested that his owners put a companion in the field with him. They borrowed a 20-year-old Miniature Horse mare. Three days after her arrival the gelding went down and slept almost an entire day. His collapsing episodes never returned.

Many horses need a strong female presence in the herd in order to feel comfortable enough to sleep explains Bertone. "Equine society is very matriarchal. Mares are responsible for the day-to-day well-being of the herd and act as sentinels watching over other horses as they rest," he says. "Males can fill this role,but in general this is a mare's job. There have actually been studies that show that horses will look to the nearest mare before they lie down. These horses feel safer knowing that a mare is on lookout. They don't seem to trust geldings or even stallions as much."

In fact, he says, the practice of separating mares and geldings into different fields may lead to very tired geldings.

In a similar case Bertone was called to consult on a 10-year-old Paint gelding who frequently came near collapse when he was placed in cross ties. The physical exam turned up nothing but a discussion of the horse's social arrangements revealed that the gelding had previously been pastured with only one other horse. When a younger gelding was added to the group the Paint would challenge and chase the new horse almost constantly. Even under saddle the Paint's attitude changed--he became disagreeable and unpredictable, tossing his head and using evasive behaviors.

The collapsing episodes and behavior problems led the owner to sell the Paint shortly after her first consultation with Bertone. Curious about what happened to the horse he followed up with the new owner who reported no problems behavioral or otherwise. The only odd thing, she said, was how much the gelding slept during his first week in his new home. She also mentioned that the Paint was now part of a herd where a mare was at the top of the pecking order but he had adapted readily and stuck close to her all day.

This horse, Bertone theorizes, was exhausting himself trying to enforce the pecking order of his old herd. When he changed herds and no longer felt the need to dominate, he was able to relax and get some rest.

Identifying Sleep Troubles

These cases illustrate the need to consider sleep deprivation as a possible cause of apparent collapse or perhaps other unexplained sluggishness says Bertone. "I really discounted the importance of sleep in horses until I connected the fact that these horses, commonly labeled as narcoleptics, actually don't fit the bill for that disease. Also I became concerned that horses that presented with these clinical signs were suspected of having EPM and it would nearly be impossible for EPM to be associated with the changes necessary to cause these clinical signs. Unfortunately and typically these horses have been diagnosed with all sorts of things, usually EPM or HYPP, and treated to no avail."

Bertone has developed a simple diagnostic protocol involving three basic questions to help identify otherwise healthy horses who are suffering from sleep deprivation.

1. Have you seen your horse roll lately? If the owner says "no" or can't recall, Bertone investigates the possibility of musculoskeletal or abdominal pain. If the owner says "yes," he investigates behavioral issues.

2. Has anything changed in the horse's social situation? "If pain isn't the cause, we turn to the social setting," says Bertone. "The goal is to identify why he might not feel comfortable enough to lie down." Often he says adding another horse to the herd or removing an aggressive animal fixes the problem: "You have to experiment with groupings and the environment a bit and give it a few days after each change to see how it works." The horse's physical surroundings are also a consideration. "Maybe the horse needs a bigger stall more bedding or even a different horse in the next stall to be comfortable," he adds.

3. Is the horse's environment noisy or somehow disturbing? "One case of sleep loss was the result of a nearby highway under 24-hour repair for the previous seven days," says Bertone. In that case moving the horse to a paddock farther away from the construction solved the problem. Whether the disturbance is during the night or day is of little consequence, says Bertone, because a horse's sleep patterns are not dependent upon time of day.

The trial-and-error process involved in resolving sleep deprivation problems can take a few weeks but when you hit upon the solution the results are remarkable: "In these cases the horses are just so relieved to finally be able to get some sleep," says Bertone. "They'll spend a few days 'catching up' and then go back to a normal sleep pattern. And they are happier and healthier for it which only makes sense: Who doesn't need and appreciate a good night's sleep or a good midday nap?"

This article originally appeared in the February 2007 issue of EQUUS magazine.