Sleep is an essential part of the equine ethogram and is an important maintenance function. By understanding the characteristics of equine sleep, we can identify when there may be a problem. Although sleep now has a well-known physiology, there is not yet a clear understanding of the purpose of sleep; however, it is suspected to be a vital body maintenance behaviour as well as helping with the consolidation of memories (Greening et al., 2020).
It is not uncommon for vets to diagnose horses with narcolepsy when they are reported to be suffering from sleep attacks or sleep deprivation. Yet rather than being a neurological disorder, “narcolepsy” in adult horses is likely to be a rapid eye movement (REM) sleep deficiency caused by recumbent sleep deprivation (Fuchs et al., 2016). This can be caused by a variety of factors such as pain, illness, management inadequacies or ethological needs not being appropriately met.
Sleep cycles
There are five stages of sleep – alert wakefulness, diffuse drowsiness, intermediate period, slow-wave sleep and paradoxical sleep (Dallaire, 1986). Horses are polyphasic sleepers requiring a total of between 2.5 and 5 hours of sleep in each 24-hour period (Houpt, 1980; Dallaire, 1986; Aleman et al., 2008; Zanker et al., 2021), which is around 15 percent of the total time budget. Sleep is mainly nocturnal and sleep phases are typically distributed between the hours of 8pm and 5am, with the most slow-wave and paradoxical sleep phases occurring between the hours of midnight and 4am (Dallaire, 1986). There is also a tendency for horses who are not stabled to sleep between the hours of 12pm and 2pm; however, paradoxical sleep has not been typically observed during this afternoon nap time (Dallaire, 1986).
[Horses] must lie down to be able to enter paradoxical sleep and if they are unable or unwilling to lie down they may suffer from sleep deprivation
Before entering a phase of paradoxical sleep, the horse will briefly awaken during the intermediate period while in a standing position to check the safety of their environment, then they will lie down and enter a phase of slow-wave sleep. If the horse is comfortable and confident in their environment, they will then move into lateral recumbency and enter a phase of paradoxical sleep. Horses are not able to get the full range of sleep while standing up – they must lie down to be able to enter paradoxical sleep and if they are unable or unwilling to lie down they may suffer from sleep deprivation.
Electrical brain activity during paradoxical sleep consists of primarily similar brain waves to those seen during alert wakefulness (Dallaire, 1986). During paradoxical sleep, muscle atonia and bursts of rapid eye movement (REM) occur. Rapid movements of the ears as well as facial and labial muscular twitching often occur during these bursts, and they may be accompanied by contractions of the limbs and even vocalisation in some cases.
Recumbent sleep deprivation
If horses are physically unable to lie down or choose not to for any reason then recumbent sleep deprivation occurs, causing a substantial impact on the health and quality of life of the individual. Recumbent sleep deprivation can cause serious injuries as often the individuals will fall into paradoxical sleep while standing and subsequently the complete muscle relaxation causes them to collapse (Figure 1).
A recent study found injury occurrence in more than 90 percent of the observed horses suffering from recumbent sleep deprivation, with the most common injuries found on the knees and fetlocks where they had dropped quickly to the floor as their legs buckled (Fuchs et al., 2019; Figure 2). Head injuries and hock injuries are also often seen in association with this condition (Fuchs et al., 2019).
These collapses can range greatly in severity between individuals, from a light swaying to very sudden complete collapse. When horses have these collapses, they are most likely to occur during the night, so they are something owners may not observe themselves. Instead, they may find some tell-tale signs in the morning such as new unexplained injuries. For this reason, cases may be left undetected for months or even years (Coomer and Fouche, 2010), particularly in the absence of overnight surveillance.
What can cause sleep deprivation in horses?
Environmental stressors can be a contributing factor to the development of sleep deprivation and may include things such as extreme temperatures, lack of social stability, lack of social contact, unfamiliar surroundings (which is likely to cause them to feel unsafe in their environment) and inappropriate housing, among others (Bertone, 2006, 2022; Coomer and Fouche, 2010; Fuchs et al., 2016).
Horses rely on having a sentinel to stand watch over them when they sleep. As a prey species, this is an important safety factor and as a result, horses who are not in a stable herd of companions may find it difficult to feel safe enough to lie down to sleep. Horses who are repeatedly disturbed or pushed around by other horses may similarly feel too vulnerable to lie down, thus preventing them from achieving paradoxical sleep. The introduction of new horses or the removal of horses from a stable herd may disrupt the herd dynamics and have a negative effect on the amount of sleep the individuals are getting.
The introduction of new horses or the removal of horses from a stable herd may disrupt the herd dynamics and have a negative effect on the amount of sleep the individuals are getting
Hospital patients and horses taken to competitions with overnight stays may also be at risk of suffering from sleep deprivation. This is due to the busy, stressful and unfamiliar environments that they find themselves in. Changes of environment are stressful for horses – when horses move to a new yard they will typically not sleep at all for the first few days and it is believed that they have reduced sleep for up to a month while they try to settle in (Dallaire, 1986; Budiansky, 1998; Ruckebusch et al., 1970). This is one of the reasons why it is not recommended to start riding a horse as soon as they arrive at their new home as there are lots of factors that may cause this to be unsafe, with sleep deprivation being just one.
Signs of sleep deprivation
Behavioural signs of sleep deprivation may not be obvious without overnight video facilities as sleep crashing episodes are much more likely to happen during the night. Signs will also vary between individuals, but they may include:
- Recurrent carpal and fore fetlock abrasions (caused by collapse or partial collapse)
- Lack of evidence of lying down (eg no bedding in the tail or on the body)
- Increased daytime drowsiness
- Weight loss
- Poor performance
- Increased irritability and hypervigilance
- Complete or partial collapse
- Unexplained facial injuries, particularly to the mouth and muzzle area (as a result of collapse)
It should be noted that many sleep-deprived horses will continue to roll on a daily basis, despite not lying down. This is important for owners to understand so that they do not misinterpret signs such as mud on the horse’s coat as proof that they have been lying down.
It should be noted that many sleep-deprived horses will continue to roll on a daily basis, despite not lying down
What can be done?
Diagnosis and monitoring
Setting up a camera to video the horse during times when people are not present can be crucial in helping with the diagnosis of sleep disorders. If it is not possible for cameras to be used 24 hours a day, the most beneficial time to gain behaviour recordings is during the night between 8pm and 6am. Behaviours are often seen during these hours that may not be observed at other times of day and pain behaviours are more likely to be expressed during these quiet hours when there are fewer environmental distractions (Figure 3).
Owners of horses suspected to be suffering from sleep deprivation should be encouraged to keep a daily diary including:
- When the events are occurring
- In what situations they occur
- Whether the horse is stabled or turned out
- Whether the herd has recently been disrupted in any way
- If the horse has been travelling or competing
- If there are any medical problems
- Whether there have been any other potential environmental stressors at the time
Dr Joe Bertone, DVM, suggests considering whether there has been a potentially stressful event approximately two weeks before the onset of sleep crashing behaviour – in his experience, where the cause for sleep crashing is anxiety-based, the trigger can often be identified as something that happened roughly two weeks prior to the onset of the behaviour (Bertone, 2022).
Prevention
Where a sleep disorder is suspected, owners should be advised to ensure that the stable and any concrete areas are well padded with rubber matting on the floors and walls. A thick bed should also be provided to prevent injury where possible. Some horses may benefit from wearing padded boots on their forelegs to prevent recurrence of abrasions while further investigations take place.
Examination and identification
Clinical examination to rule out any cardiac or neurological factors for collapse is essential before further investigation into whether any causes of pain can be identified (de Klerk, 2019). If a thorough veterinary investigation is unable to identify any painful conditions, then the recruitment of an equine behaviour consultant may be necessary. Equine behaviour consultants can help the owner to evaluate and adjust, where necessary, the management and environment of the horse to address any potential causes of environmental insecurity. However, it is crucial to recognise that just because a cause of pain has not been identified, it does not necessarily mean that the horse is not experiencing pain, and pain should always remain a possible differential for cases of sleep deprivation. Where a painful condition is the cause of sleep deprivation, sleep behaviour should return to normal soon after successful treatment of the condition. Where pregnancy-related discomfort is the cause, problems typically resolve rapidly following the birth of the foal (de Klerk, 2019).
It is crucial to recognise that just because a cause of pain has not been identified, it does not necessarily mean that the horse is not experiencing pain
When sleep deprivation occurs due to environmental or anxiety-based factors, identification of the exact cause is necessary to enable the owner to make the necessary alterations to the horse’s life. If anxiety or fear are relevant in the development or continuation of sleep deprivation it may be necessary to discuss the case with an equine veterinary behaviourist to consider the use of psychopharmacology alongside a behavioural modification programme provided by the behaviourist.
Conclusion
Every horse suffering from sleep deprivation will have their own individual reason for not lying down to sleep. Owning a sleep-deprived horse is an emotionally exhausting experience and can have detrimental effects on the psychological well-being of the owner as well as being an obvious welfare and quality-of-life concern for the horse. So a collaborative and methodical process of elimination between veterinary surgeons, equine behaviourists and owners can be a beneficial approach to providing solutions for these often frustrating and complex cases.
