February 2008 • VOLUME 30 • © HORSES For LIFE™ Magazine
The Need for Sleep
The Need for Sleep
In barns across Europe, in city after city where literally hundreds of thousands of horses lived out their lives as taxis, laundry haulers, coal deliverers, delivering everything from milk to the goods that the industrial factories required on a daily basis, they were your bus, your semi-truck, your car to run to the store, they literally were everywhere. And in the process, the barns needed to fit in as many horses as possible. Tie stalls are an obvious answer to the problem. The horses out working all day long, at night in stables in the cities and in barns in the country, needed little room to move around as they were well tired and only needed sleep. Even today many barns incorporate tie stalls.
One of the most fascinating facts to many is how the horse can sleep standing up. It almost appears magical. The science behind the magic is the horse’s ability to lock his front knees and to take one ligament in his back leg and hook it over his hock. A stabilizer or locking mechanism if you wish, that enables the horse to stand while sleeping.
Because of this mechanism, and it seems some horses' preference for sleeping standing up - that for years, perhaps even centuries, it was thought that there were some horses that really didn’t like to lie down to sleep or even need to lie down to sleep. It made so much sense. After all, what prey animal wants to be lying down, making it easier for the predators to attack? Making tie stalls a natural outcome.
WHAT WE KNOW ABOUT SLEEP IN HORSES
Well, it actually turns out that horses need three different types of sleep, for a total of about four to five hours of sleep a day.
“Sleep is commonly defined as a period of immobility in which individuals seem unresponsive to their environment. Sleep now is accepted as a behavior with clear physiological necessities that are not fully understood, but which when lacking have clear dysfunctional effects. As a component of a single day, it is estimated that horses require 2 hours of drowsiness, 3 hours of slow wave sleep, less than 1 hour of paradoxical sleep, with 4 hours of total sleep time. There is no definitive explanation for the necessity or the function of sleep in horses, or for that matter in all other species.”
THE STATES OF SLEEP
“Sleep is divided into several different states that include slow-wave sleep (SWS), paradoxical sleep (PS), and REM sleep. During REM sleep, the EEG pattern resembles that of wakefulness, despite the fact that the individual is deeply asleep.”
According to J.J. Bertone of the College of Veterinary Medicine of the University of Health Sciences Pomona, California, the need for, or the lack of sleep “are not broadly widespread areas of concern in veterinary medicine and surgery. There is a seemingly traditional unconcern and dismissal of the clinical ramifications of sleep on performance and clinical outcome associated with these areas of essential, sustaining, recuperative, physiology in the veterinary clinical literature.”
Doing without some of the different states of sleep means that the horse could be suffering from sleep deprivation. Horses have three different basic positions for sleep. It is important to know the three basic reasons the horse might not take one of the positions needed to get the type of sleep that they need.
Horses sleep in three different positions. Standing up, lying down but upright on their sternum, referred to as sternal recumbency, and lying down on their sides, referred to as lateral recumbence. The different types of sleep tend to happen in these different positions.
Paradoxical sleep tends to happen in a lateral recumbent position with the horse lying down and the neck tonicity at a minimum. But there are many reasons why the horse will not lie down to take the sleep that he needs.
Bertone tells of a few of the
specific reasons why the horse will not lie down, each of which can cause sleep deprivation:
“Horses need a period of recumbent sleep that can be avoided for several days, but eventually must be taken. Recumbent sleep deprivation has a specific behavior where horses become drowsy, begin to enter paradoxical sleep, partially collapse, then arouse and instead of lying down, they go through the same cycle. The author places cases of excessive drowsiness into three categories.
Pain Associated Excessive Drowsiness is defined as those cases where horses do not lie down because it is painful to do so, or it is painful to rise, or both. Most often, these horses have musculoskeletal issues that create pain or mechanical difficulties on attempts at recumbency or rising to stand. In addition, horses have episodes of excessive sleepiness in association with thoracic or abdominal pain that is elicited with recumbency or on attempting to stand.
Environmental Insecurity–Associated Excessive Drowsiness is a condition in which horses do not lie down because they are psychologically uncomfortable doing so, because horses will only lie down when they are environmentally comfortable. One can assume that this stems from horses as prey and also herd behavior. The author has managed several horses with this syndrome by adding a friend to the pasture or stall, or moving the horse to an area where there are more horses, or removing an aggressive horse from a group where several horses may have the problem. This would also include horses in an insecure environment that may just require a larger stall or paddock, or those horses near loud harassing noise, like fireworks or speedways.”
We also need to consider that each horse is an individual and what works for one horse may not work for another. While one horse feels secure in a larger box stall and willing to lie down on their side, another may not. We also need to remember that sternal recumbency, or lying down but upright resting on the sternum, is not the same as actually lying down and resting out completely on the side.
We need to ensure that the horse feels safe enough to lie down. Horses as prey animals are genetically bred for thousands of years to rest on their side when there is another horse looking out for them - a horse that is the sentinel while they are resting.
While barn managers do make every effort to put horses together that get along well enough not to harm each other, space limitations and the number of horses often mean that horses are put together in a smaller area, and with horses, while they may for the most part get along well enough not to hurt each other, truly don’t get along.
What does this mean for your horse?
This means that your horse may not feel comfortable enough or safe enough to lie down and completely stretch out. It is one thing to trust another horse while you are on your feet and can defend yourself, another to trust that same horse while you are asleep, on the ground and unable to defend yourself.
We also need to remember that rest and sleep are two different things. An animal can rest without sleeping.
“Sleep can be altered by environmental and physical factors. Horses assume recumbency in a familiar environment. When moved to an unfamiliar environment, sleep can be adversely affected for several days, depending on the new environment.
When subjects are put outdoors, it may be 1 or 2 days before they first lie down. This habituation may be socially facilitated. If one horse is familiar with the new environment, it will lie down and the others will follow. It is felt that the dominant animal is usually the first to lie down.
Observing wild ponies indicated that recumbent resting behavior occupies 16% of a horse’s time. This behavior reaches a peak between 12:00 AM and 4:00 AM. All ponies were never seen to be lying down simultaneously. However, this contrasts with a study with feral Appaloosa horses where the entire herd was seen recumbent at one time during each day.”
THE STATES OF SLEEP
Sleep is divided into several different states that include slow-wave sleep (SWS), paradoxical sleep (PS), and REM sleep. During REM sleep, the EEG pattern resembles that of wakefulness, despite the fact that the individual is deeply asleep.
PHYSIOLOGIC CHARACTERISTICS OF WAKEFULNESS AND SLEEP
“Wakefulness EEG is characterized by slow-amplitude fast waves or LVFA (low-voltage fast-activity). Eye movements are frequent, rapid and irregular. Neck muscular tone is high but largely variable, depending on the position of the head. During diffuse wakefulness, as the horse becomes drowsy, LVFA waves are partially replaced by large slow waves. As the horse falls into sleep, there is an increase in high-voltage slow-activity (HVSA) waves. Rapid eye movements are replaced by slow and mostly horizontal movements. Neck and other muscular tone are moderately high, however with very regular amplitude. During SWS, the EEG is characterized by HVSA. Cortical electrical activity is synchronized. EOG indicate that eye movements are very scarce, slow and horizontal. The eyelids are often only slightly opened. Neck EMG evidences reduced muscular activity which nearly disappears if the head is resting against a support. In paradoxical sleep, brain electrical activity consists of beta waves similar to alert wakefulness. Muscular tone has completely disappeared (corticospinal disassociation). At irregular intervals during PS, there is REM usually associated with pinnal, facial, limb muscular twitching, and soft whinnying.
During SWS, there is a tonic increase in the parasympathetic outflow with a slight attenuation of sympathetic activity. The cardiac and respiratory rates decrease. The reduction in rates of both activities is significant from wakefulness to SWS and the variability is reduced. PS is associated with slightly lower respiratory and cardiac rates than those seen during SWS. However, variability is increased, even more than during wakefulness. This is due to the presence of arrhythmias. There are periods of tachycardia followed by extreme bradycardia. Respiration is characterized by sudden acceleration and deceleration and even pauses. This is followed by bursts of REM. Variations in arousal threshold are evident from drowsiness to PS. Paradoxical sleep is the deepest level of sleep; arousal threshold is at its maximum.
SWS always precedes PS. Sleep cycles (SC) are measured from the onset of one PS episode to the onset of the next. Multiple phases may occur throughout one 24-hour period and there may be more than one sleep cycle during each phase. In horses, the mean duration of a sleep cycle is approximately 15 minutes. The mean durations for PS and SWS are 4.2 and 6.4 minutes, respectively. The difference is represented by drowsiness and by the presence of an intermediary phase, which occurs between SWS and PS in almost 30% of sleep cycles. This intermediary phase looks like drowsiness. During this period, the arousal threshold is closer to that of diffuse wakefulness than that of drowsiness. This state may represent a partial awakening before entering deep sleep. The fact that the animal does not enter PS directly from SWS can be thought of as a protective mechanism. This allows a horse to avoid entering deepest sleep without considering the security of the environment. In domesticated species, this phenomenon has only been documented in rodents. Total sleep time (TST) averages 3 to 5 hours per day (15%). Sleep occurs in 5 to 7 phases each night that last 30 to 60 minutes. PS last for about 1 hour per day. The PS/TST ratio usually takes 21 to 25% of the day. It is well known that TST and the PS/TST ratio are age determined. Observations have revealed that lying flat occupied 15% of the time after birth and only 2% after weaning. Due to the close relationship between lateral recumbency and PS, we may assume that this change is related to a decrease in PS time. For stabled horses, sleep is mainly nocturnal and distributed between 8:00 PM to 5:00 AM. Sleep phases are separated by eating or dozing periods. The maximum concentration of SWS and PS is from midnight to 4:00 AM. Horses are often seen sleeping between 12:00 PM and 2:00 PM, but not regularly in stabled conditions. The time horses will sleep depends mainly on the level of activity around the stall. Most often PS is not observed during the afternoon nap.
The drowsy horse has its eyelids partially opened and its head hangs slightly lower or even with its wither. As the horse enters SWS, the head gradually falls. When there is environmental confidence the horse lies down.
The horse's brain enters drowsiness again and very rapidly goes into SWS. After a short time, a PS episode often occurs. When it does, the horse lies in lateral recumbency. If this isn’t possible, the head and the body will rest against an object.
In PS, muscular tone disappears. In drowsiness and SWS, the attitude of the animal determines the intensity of neck muscular activity. In a standing subject, during drowsiness, the head lowers with progressive reductions in tonic neck muscle activity and culminating with disappearance if the head comes close to the ground. In the recumbent animal, cervical muscular tone is also always present, even if the head is resting against the ground, but it may be reduced to a minimum.
Horses usually stand with one hindleg partially flexed during drowsiness. It is possible to observe SWS in the standing animal, but if well adapted to its environment horses become recumbent. SWS will largely occur in sternal recumbency. It may happen on rare occasions in lateral recumbency.
Although the horse is known for its ability to sleep while standing because of anatomic adaptations of its limbs, it is unable to complete a sleep cycle without lying down to enter PS. If a horse is put in a straight stall where it is physically impossible for it to lie down, it will be PS sleep-deprived. In this situation, SWS may increase in duration, but this doesn’t compensate for PS deprivation. As soon as the animal can become recumbent posture, it will go into total PS time. The increase in PS duration will last for 2 to 3 nights and may be twice the normal duration.
It is likely that the benefit of the ability to stand while sleeping may be related to the niche of the horses; prey and the protection against predators. It is also of interest to consider that the energy cost of standing in horses during drowsiness and SWS is decreased by the presence of the stay apparatus. In PS, which is the deepest sleep, horses can still be aroused easily by any strange noise and can stand quickly. In summary, although each state may occur in more than one posture, PS normally occurs in lateral recumbency, SWS in sternal recumbency, and drowsiness in the standing posture.”
For those that are responsible for the care of horses, from stable managers to owners with horses in their own back yard, we need to ensure that we provide our horses with an environment where they feel safe and comfortable, where they will be able to lie down stretched out and get the type of sleep that they need so that they do not become sleep deprived. We need to ensure that if our horses are in a smaller pasture that they are with horses that they feel safe with.
All Quotes from:
NAVC Proceedings 2007, North American Veterinary Conference (Eds). Publisher: NAVC
Sleep Deprivation - Not Narcolepsy - In Horses
J.J. Bertone
College of Veterinary Medicine, Western University of Health Sciences Pomona, CA, USA.
References
1. Lunn DP, Cuddon PA, Shaftoe S, Archer RM. Familial occurrence of narcolepsy in miniature horses. Equine Vet J 1993;25(6):476-7.
2. Mignot E. Narcolepsy: Pharmacology, pathophysiology, and genetics. In: Kryger MH, Roth T, Dement WC (eds.): Principles and Practices of Sleep Medicine. Philadelphia: Elsevier Saunders 2005, pp 761-779.
3. Guilleminault C, Fromherz S. Narcolepsy: Diagnosis and management. In: Kryger MH, Roth T, Dement WC (eds.): Principles and Practices of Sleep Medicine. Philadelphia: Elsevier Saunders, 2005, pp 780-790.
4. Dallaire A. Rest behavior. Vet Clin North Am Equine Pract 1986;2(3):591-607.
5. Karmanova IG, Lazare SG. Stages of sleep evolution (facts and hypotheses). Waking and Sleeping 1979;3:137-147.
6. Winchester CF. The energy cost of standing in horses. Science 1943;97(2505):24.
7. Ruckebusch Y. The relevance of drowsiness in the circadian cycle of farm animals. Anim Behav 1972;20:637-643.
8. Keiper RR, Keenan MA. Nocturnal activity patterns of feral ponies. J Mammal 1980;61(1):116-118.
9. Stebbins MC. Social organization in free-ranging Appaloosa horses. Unpublished MS Thesis, Pocatello, Idaho State University, 1974.
“Horses need a period of recumbent sleep that can be avoided for several days, but eventually must be taken. Recumbent sleep deprivation has a specific behavior where horses become drowsy, begin to enter paradoxical sleep, partially collapse, then arouse and instead of lying down, they go through the same cycle. The author places cases of excessive drowsiness into three categories.
Pain Associated Excessive Drowsiness is defined as those cases where horses do not lie down because it is painful to do so, or it is painful to rise, or both. Most often, these horses have musculoskeletal issues that create pain or mechanical difficulties on attempts at recumbency or rising to stand. In addition, horses have episodes of excessive sleepiness in association with thoracic or abdominal pain that is elicited with recumbency or on attempting to stand.
Environmental Insecurity–Associated Excessive Drowsiness is a condition in which horses do not lie down because they are psychologically uncomfortable doing so, because horses will only lie down when they are environmentally comfortable. One can assume that this stems from horses as prey and also herd behavior. The author has managed several horses with this syndrome by adding a friend to the pasture or stall, or moving the horse to an area where there are more horses, or removing an aggressive horse from a group where several horses may have the problem. This would also include horses in an insecure environment that may just require a larger stall or paddock, or those horses near loud harassing noise, like fireworks or speedways.”
We also need to consider that each horse is an individual and what works for one horse may not work for another. While one horse feels secure in a larger box stall and willing to lie down on their side, another may not. We also need to remember that sternal recumbency, or lying down but upright resting on the sternum, is not the same as actually lying down and resting out completely on the side.
We need to ensure that the horse feels safe enough to lie down. Horses as prey animals are genetically bred for thousands of years to rest on their side when there is another horse looking out for them - a horse that is the sentinel while they are resting.
While barn managers do make every effort to put horses together that get along well enough not to harm each other, space limitations and the number of horses often mean that horses are put together in a smaller area, and with horses, while they may for the most part get along well enough not to hurt each other, truly don’t get along.
What does this mean for your horse?
This means that your horse may not feel comfortable enough or safe enough to lie down and completely stretch out. It is one thing to trust another horse while you are on your feet and can defend yourself, another to trust that same horse while you are asleep, on the ground and unable to defend yourself.
We also need to remember that rest and sleep are two different things. An animal can rest without sleeping.
“Sleep can be altered by environmental and physical factors. Horses assume recumbency in a familiar environment. When moved to an unfamiliar environment, sleep can be adversely affected for several days, depending on the new environment.
When subjects are put outdoors, it may be 1 or 2 days before they first lie down. This habituation may be socially facilitated. If one horse is familiar with the new environment, it will lie down and the others will follow. It is felt that the dominant animal is usually the first to lie down.
Observing wild ponies indicated that recumbent resting behavior occupies 16% of a horse’s time. This behavior reaches a peak between 12:00 AM and 4:00 AM. All ponies were never seen to be lying down simultaneously. However, this contrasts with a study with feral Appaloosa horses where the entire herd was seen recumbent at one time during each day.”
THE STATES OF SLEEP
Sleep is divided into several different states that include slow-wave sleep (SWS), paradoxical sleep (PS), and REM sleep. During REM sleep, the EEG pattern resembles that of wakefulness, despite the fact that the individual is deeply asleep.
PHYSIOLOGIC CHARACTERISTICS OF WAKEFULNESS AND SLEEP
“Wakefulness EEG is characterized by slow-amplitude fast waves or LVFA (low-voltage fast-activity). Eye movements are frequent, rapid and irregular. Neck muscular tone is high but largely variable, depending on the position of the head. During diffuse wakefulness, as the horse becomes drowsy, LVFA waves are partially replaced by large slow waves. As the horse falls into sleep, there is an increase in high-voltage slow-activity (HVSA) waves. Rapid eye movements are replaced by slow and mostly horizontal movements. Neck and other muscular tone are moderately high, however with very regular amplitude. During SWS, the EEG is characterized by HVSA. Cortical electrical activity is synchronized. EOG indicate that eye movements are very scarce, slow and horizontal. The eyelids are often only slightly opened. Neck EMG evidences reduced muscular activity which nearly disappears if the head is resting against a support. In paradoxical sleep, brain electrical activity consists of beta waves similar to alert wakefulness. Muscular tone has completely disappeared (corticospinal disassociation). At irregular intervals during PS, there is REM usually associated with pinnal, facial, limb muscular twitching, and soft whinnying.
During SWS, there is a tonic increase in the parasympathetic outflow with a slight attenuation of sympathetic activity. The cardiac and respiratory rates decrease. The reduction in rates of both activities is significant from wakefulness to SWS and the variability is reduced. PS is associated with slightly lower respiratory and cardiac rates than those seen during SWS. However, variability is increased, even more than during wakefulness. This is due to the presence of arrhythmias. There are periods of tachycardia followed by extreme bradycardia. Respiration is characterized by sudden acceleration and deceleration and even pauses. This is followed by bursts of REM. Variations in arousal threshold are evident from drowsiness to PS. Paradoxical sleep is the deepest level of sleep; arousal threshold is at its maximum.
SWS always precedes PS. Sleep cycles (SC) are measured from the onset of one PS episode to the onset of the next. Multiple phases may occur throughout one 24-hour period and there may be more than one sleep cycle during each phase. In horses, the mean duration of a sleep cycle is approximately 15 minutes. The mean durations for PS and SWS are 4.2 and 6.4 minutes, respectively. The difference is represented by drowsiness and by the presence of an intermediary phase, which occurs between SWS and PS in almost 30% of sleep cycles. This intermediary phase looks like drowsiness. During this period, the arousal threshold is closer to that of diffuse wakefulness than that of drowsiness. This state may represent a partial awakening before entering deep sleep. The fact that the animal does not enter PS directly from SWS can be thought of as a protective mechanism. This allows a horse to avoid entering deepest sleep without considering the security of the environment. In domesticated species, this phenomenon has only been documented in rodents. Total sleep time (TST) averages 3 to 5 hours per day (15%). Sleep occurs in 5 to 7 phases each night that last 30 to 60 minutes. PS last for about 1 hour per day. The PS/TST ratio usually takes 21 to 25% of the day. It is well known that TST and the PS/TST ratio are age determined. Observations have revealed that lying flat occupied 15% of the time after birth and only 2% after weaning. Due to the close relationship between lateral recumbency and PS, we may assume that this change is related to a decrease in PS time. For stabled horses, sleep is mainly nocturnal and distributed between 8:00 PM to 5:00 AM. Sleep phases are separated by eating or dozing periods. The maximum concentration of SWS and PS is from midnight to 4:00 AM. Horses are often seen sleeping between 12:00 PM and 2:00 PM, but not regularly in stabled conditions. The time horses will sleep depends mainly on the level of activity around the stall. Most often PS is not observed during the afternoon nap.
The drowsy horse has its eyelids partially opened and its head hangs slightly lower or even with its wither. As the horse enters SWS, the head gradually falls. When there is environmental confidence the horse lies down.
The horse's brain enters drowsiness again and very rapidly goes into SWS. After a short time, a PS episode often occurs. When it does, the horse lies in lateral recumbency. If this isn’t possible, the head and the body will rest against an object.
In PS, muscular tone disappears. In drowsiness and SWS, the attitude of the animal determines the intensity of neck muscular activity. In a standing subject, during drowsiness, the head lowers with progressive reductions in tonic neck muscle activity and culminating with disappearance if the head comes close to the ground. In the recumbent animal, cervical muscular tone is also always present, even if the head is resting against the ground, but it may be reduced to a minimum.
Horses usually stand with one hindleg partially flexed during drowsiness. It is possible to observe SWS in the standing animal, but if well adapted to its environment horses become recumbent. SWS will largely occur in sternal recumbency. It may happen on rare occasions in lateral recumbency.
Although the horse is known for its ability to sleep while standing because of anatomic adaptations of its limbs, it is unable to complete a sleep cycle without lying down to enter PS. If a horse is put in a straight stall where it is physically impossible for it to lie down, it will be PS sleep-deprived. In this situation, SWS may increase in duration, but this doesn’t compensate for PS deprivation. As soon as the animal can become recumbent posture, it will go into total PS time. The increase in PS duration will last for 2 to 3 nights and may be twice the normal duration.
It is likely that the benefit of the ability to stand while sleeping may be related to the niche of the horses; prey and the protection against predators. It is also of interest to consider that the energy cost of standing in horses during drowsiness and SWS is decreased by the presence of the stay apparatus. In PS, which is the deepest sleep, horses can still be aroused easily by any strange noise and can stand quickly. In summary, although each state may occur in more than one posture, PS normally occurs in lateral recumbency, SWS in sternal recumbency, and drowsiness in the standing posture.”
For those that are responsible for the care of horses, from stable managers to owners with horses in their own back yard, we need to ensure that we provide our horses with an environment where they feel safe and comfortable, where they will be able to lie down stretched out and get the type of sleep that they need so that they do not become sleep deprived. We need to ensure that if our horses are in a smaller pasture that they are with horses that they feel safe with.
All Quotes from:
NAVC Proceedings 2007, North American Veterinary Conference (Eds). Publisher: NAVC
Sleep Deprivation - Not Narcolepsy - In Horses
J.J. Bertone
College of Veterinary Medicine, Western University of Health Sciences Pomona, CA, USA.
References
1. Lunn DP, Cuddon PA, Shaftoe S, Archer RM. Familial occurrence of narcolepsy in miniature horses. Equine Vet J 1993;25(6):476-7.
2. Mignot E. Narcolepsy: Pharmacology, pathophysiology, and genetics. In: Kryger MH, Roth T, Dement WC (eds.): Principles and Practices of Sleep Medicine. Philadelphia: Elsevier Saunders 2005, pp 761-779.
3. Guilleminault C, Fromherz S. Narcolepsy: Diagnosis and management. In: Kryger MH, Roth T, Dement WC (eds.): Principles and Practices of Sleep Medicine. Philadelphia: Elsevier Saunders, 2005, pp 780-790.
4. Dallaire A. Rest behavior. Vet Clin North Am Equine Pract 1986;2(3):591-607.
5. Karmanova IG, Lazare SG. Stages of sleep evolution (facts and hypotheses). Waking and Sleeping 1979;3:137-147.
6. Winchester CF. The energy cost of standing in horses. Science 1943;97(2505):24.
7. Ruckebusch Y. The relevance of drowsiness in the circadian cycle of farm animals. Anim Behav 1972;20:637-643.
8. Keiper RR, Keenan MA. Nocturnal activity patterns of feral ponies. J Mammal 1980;61(1):116-118.
9. Stebbins MC. Social organization in free-ranging Appaloosa horses. Unpublished MS Thesis, Pocatello, Idaho State University, 1974.