Understanding Equine Sleep Deprivation
Horses can avoid paradoxical sleep for several days; however, the animal must eventually lie down to cycle through all the stages of sleep. It's when a horse passes the "several day" mark without paradoxical sleep that sleep deprivation kicks in.
Today's culture fuels busy lifestyles with dwindling opportunities to sleep, so it should come as no surprise that a 2011 Center for Disease Control study estimated more than one third of American adults suffer from sleep deprivation. But did you know horses can suffer sleep deprivation as well? At the 2012 Western Veterinary Conference, held Feb. 19-23 in Las Vegas, Nev., one researcher presented an overview of this important, but not fully understood, equine health condition.
Before delving into sleep deprivation, however, it's important to understand sleep in general. Joe Bertone, DVM, MS, Dipl. ACVIM, professor of equine medicine at Western University of Health Sciences in Pomona, Calif., reviewed with the veterinary audience the basics of equine sleep.
What is Sleep and What Influences It?
"Sleep is commonly defined as a period of immobility in which individuals seem unresponsive to their environment," Bertone explained. But even though it's fact everyone needs sleep, the reasoning behind it isn't yet clear: "Sleep is now accepted as a behavior with clear physiologic necessities that are not understood, but when lacking have clear dysfunctional impact."
So how much sleep is enough to prevent the dysfunctional impact? Bertone said that research, most of which was conducted in France in the 1960s and 70s by Dellaire and Ruckebusch, estimates that in a day horses fulfill approximately:
- Two hours of diffuse drowsiness (during which the horse stands with full weight on both front legs and one rear leg while the other rear leg is cocked, or "primed," to kick if necessary);
- Three hours of slow wave sleep (at which point the horse is either lying down in lateral or sternal recumbency [lying flat on his side or lying on his chest, respectively]); and
- Less than one hour of paradoxical sleep (during which the horse is commonly positioned on the ground, but the body is upright with head tucked to a side--normally toward the stifle). This allows for better respiration than being on a side.
"Horses can go for days without holding to this pattern," Bertone explained. "That makes evolutionary sense since horses were a migratory species. Having to take a nap may have made you someone else's dinner."
For these reasons, equine sleep patterns are often influenced by environmental and physical factors, he said. For instance, horses can be influenced to lie down for paradoxical sleep if they reside with other horses that lie down regularly. And if one horse is familiar with an environment, he explained, that horse will likely lie down and others will often follow.
However, there are also factors that influence a horse to remain on his feet. "When moved to an uncomfortable environment, sleep can be adversely affected for several days," Bertone relayed. He noted that horses can avoid paradoxical sleep for several days; however, the animal must eventually lie down to cycle through all the stages of sleep.
It's when a horse passes the "several day" mark without paradoxical sleep that sleep deprivation kicks in, he said.
Types of Sleep Deprivation and Management
"Recumbent sleep deprivation is manifested as diffuse drowsiness that moves into early slow wave sleep," Bertone explained. "Partial collapse follows with sudden arousal."
Bertone then reviewed the four categories he uses to describe sleep deprivation, or excessive drowsiness:
Pain-Associated Excessive Drowsiness--Horses in this category avoid lying down due to pain associated with lying down or standing back up, he said. Most commonly, he added, musculoskeletal pain is the culprit in these scenarios, although thoracic (chest area) and/or abdominal pain and late gestation have been known to play a role as well. In some cases, veterinarians and caretakers can manage these horses with pain-relieving drug administration and facility, footing, and bedding, Bertone said.
Monotony-Induced Excessive Drowsiness--"(This) is best exemplified by the horse in crossties being braided that begins to lower its head to the point of near collapse," Bertone described, explaining that this near collapse is believed to be the body transitioning from diffuse drowsiness to slow wave sleep. "The horse is often very comfortable in its environment, and sleep is simply induced," he added. Owners can avoid this by suspending whatever activity the horse finds monotonous and/or taking the animal for a walk, he said.
Environmental Insecurity-Associated Excessive Drowsiness--As the name suggests, this type of sleep deprivation results from a horse being psychologically uncomfortable lying down in his current environment. "In review of the over 300 cases I have seen or consulted on, this is the most common form," Bertone said. "One can assume that this stems from horses as prey and herd behavior." He suggested several management strategies for environmental insecurity associated excessive drowsiness including:
- Adding another horse--especially a mare, he noted--or another companion animal to the affected horse's stall or pasture;
- Moving the affected horse to a more heavily horse populated area;
- Removing a potentially aggressive horse from the affected horse's herd;
- Removing the affected horse from areas near "loud, harassing noises," like fireworks or train tracks; or
- Increasing stall size or leaving the horse with others in a pasture.
Dominance Displacement Excessive Drowsiness--This condition most commonly affects geldings that try to take control of a herd lacking a dominant mare. "These horses are often seen as aggressive towards other horses," Bertone said. Sleep deprivation becomes evident when affected horses are stalled or removed from the herd (i.e., when they're displaced from a position of dominance). These horses tend to be hard keepers, Bertone noted. "These horses need a dominant, no holds barred mare to put things in order," he added.
"The prognosis for the behavioral cases is case by case dependent," Bertone explained. "It's hard to tell who will respond (to management changes) and who won't."
Sometimes behavioral cases have very subtle causes that often go overlooked, such as a horse wearing (or not wearing) blankets, field geography, and weather, Bertone noted.
Bertone added that sleep-deprived horses are generally safe to ride, but it's advisable to consult a veterinarian prior to tacking up once a diagnosis has been reached.
"In my 20-year case collection of horses with signs of sleep deprivation, it's been unusual to manage a case with drugs, other than for pain relief," Bertone concluded. "An understanding of sleep, sleep deprivation, and narcolepsy makes for a clearer, more successful diagnosis and management plan."
Bertone added that he's always happy to receive e-mails regarding sleep-deprived horses. Interested parties can contact him at firstname.lastname@example.org.
About the Author
Erica Larson, news editor, holds a degree in journalism with an external specialty in equine science from Michigan State University in East Lansing. A Massachusetts native, she grew up in the saddle and has dabbled in a variety of disciplines including foxhunting, saddle seat, and mounted games. Currently, Erica competes in three-day eventing with her OTTB, Dorado.