The ability to sleep standing up enables a horse in the wild to escape predators. At the crack of a twig, a standing horse can sprint away from danger, something it could not do quickly if it were lying down.
Domestic horses rarely need to escape predators, but genetics preserve the physical characteristics that make it possible. Tendons and ligaments support the stay apparatus that enables them to sleep standing up, but the brain also plays an important role.
Rarely, a horse's stay apparatus may fail, causing it to collapse — and causing alarm in an owner who witnesses it.
“The horse's nose will get lower and lower, and they will normally start to buckle in their front legs, and their nose will hit the ground, and they will start to collapse. But then they catch themselves and they kind of wake back up for a period of time,” said Dr. Amy Johnson, assistant professor of large animal medicine and neurology at the University of Pennsylvania's New Bolton Center. “And the cycle can repeat itself, over and over and over in the course of a day.”
Some veterinarians theorize these horses have an equine version of narcolepsy, a neurological disorder that affects the control of sleep and wakefulness, but Johnson said true narcolepsy in horses is very, very rare. She, instead, attributes the phenomenon to something humans know well — sleep deprivation. It occurs when a horse is unable to lie down so its body can go into a deep sleep, called REM for the characteristic rapid eye movement during deep sleep. During REM sleep, the horse's muscles relax, enabling it to rest and revitalize.
“Horses don't need as much REM sleep as humans, but they do need two to three hours a day sleeping on the ground in order to get restful sleep,” Johnson said.
Reluctance to lie down to sleep may stem from physical or psychological issues. The horse may find it difficult or painful to get up and down, or its environment may cause anxiety. In a herd, some horses will lie down to sleep while one or two others sleep standing up, like sentries. But a horse removed from a herd and placed in solitary may feel unsafe lying down in a deep sleep. So it stands.
“After a week or maybe months, they become so sleep deprived that they begin entering a deep sleep standing up,” Johnson said. “When they go into this deep sleep, neurologically is triggered a relaxed muscle tone, which is supposed to happen after they are already lying down. And that's why they collapse because they enter this very deep sleep.”
Some of these horses will learn to protect themselves from falling down by leaning against something when they sleep standing up.
The physical stay apparatus
A horse's hind stay apparatus consists principally of the stifle and the hock. The patella (stifle) locks over the head of the femur in such a way to cause the hock joint also to lock in place. Tendons and ligaments hold the limb in that position. A horse doesn't have to lock both hind limbs; it can lock one while resting the other. This is why you may see a horse asleep standing up, with one hind foot resting on its toe. Then, after a few minutes, the horse may shift its weight to rest the other limb on its toe.
“In the forelimb, the stay apparatus is not very well formed,” said Dr. Michael Ross, professor of surgery at New Bolton Center. “It's not as intricate as the mechanism of the stifle. The forelimb is much more under muscle control. Having said that, I've seen horses buckle and fall in front because of neurological conditions, such as equine protozoal myeloencephalitis [EPM] that affects the nerves, spinal cord, and muscles.”
Ross said if a horse has normal muscle control and normal neurological status, there's no reason for it to collapse when it falls asleep. But sometimes the stay apparatus can be disrupted by an injury, such as a laceration of the patellar ligament that locks the stifle in place.
“I would worry that horse has some type of condition that causes it to lose control of its muscles temporarily, and that's why it falls. It's not that it has a problem with a single ligament,” Ross said.
Horses reluctant to lie down because of psychological issues should be observed closely to uncover the reason for their anxiety, if possible, and eliminate it. Sometimes, just a change in the layout of the paddock may make a difference, concluded Dr. Michael Glade of the University of Maryland, who studied the social sleeping behavior of horses. One of his observations was that when space is limited, a rectangular paddock is preferable to a square paddock, even when the total square footage is equal, because a rectangle provides the greatest linear separation between horses.
A horse that tends to collapse when it dozes off while standing should be examined to determine if it has a physical condition that discourages it from lying down to sleep or if it has a neurological problem. Old, arthritic horses could benefit from anti-inflammatory medication or joint supplements, in addition to offering them a deeply bedded stall to cushion their old bones while they sleep.
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