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.
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."
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."