Do horses gallop in their dreams? DR JENNIFER STEWART discusses sleep and the problems associated with not getting enough shut eye.
Sleep is imposed upon us by the needs of the brain. Even during sleep nerve cells are active – sending waves of varying frequency and size sweeping across the surface of the brain. As horses sink further into sleep, the waves become slower, larger and more synchronized as they enter short-wave sleep (SWS).
As sleep deepens, brain activity becomes more chaotic with bursts of activity in different areas of the brain cortex, the eyes, and the ear muscles. This is known as REM (rapid eye-movement) sleep and it’s when dreams occur. Behind closed lids, the eyes swing this way and that (hence the term REM). The brain seems to be seeing and hearing things that aren’t there. As it wakes up, the brain reverses back through these sleep stages.
SWS is shallow and horses can enter that sleep phase standing or lying in upright recumbency. There is muscular activity but little eye movement and the eyes may be partially closed, the head lowered to wither height or below, and the ears rotated sideways or backwards. REM sleep is only achieved when they lie flat out on their sides with complete muscle relaxation, eyelids closed and eyes moving rapidly.
REM sleep can be quite dramatic – with paddling, twitching, flared nostrils, flicking ears and blinking as they dream. In horses, SWS and REM sleep occur in five minute cycles. The usual sleep pattern for horses takes three to five hours per day, mostly between midnight and 5:00am. For solitary stabled horses, a loop of Beethoven’s 9th symphony (at 62.3 decibels) between 8:30pm and 1:30pm was found to increase the time spent in REM sleep. It’s thought the music reduced outside noises and therefore the need for the horse to stay vigilant. Less time being vigilant allowed more time for the biologically significant, more natural evolutionary behaviours of browsing and sleeping.
Although foals and young horses can maintain longer than 15 minutes in full lateral recumbency, adult horses can’t because the abdominal contents compromise respiration. Foals sleep more per day than adult horses. However, foals with skeletal pain spend less time in lateral recumbency and this may be the only sign of discomfort. Early recognition of this changed behaviour allows early intervention and treatment.
Horses can sleep standing due to the ‘stay apparatus’ which allows their body to be supported without active muscular control. To prevent the shoulder from collapsing into a flexed position, a tendon runs from the top of the elbow to below the knee, letting the biceps muscle relax while locking the legs into an upright position. During drowsiness and SWS the stay apparatus is activated, the head is lowered, eyes closed and one hind leg rests. In very relaxing situations, like grooming, sun-bathing or standing quietly in cross-ties, horses may sneak a few episodes of sleep.
Resting or drowsiness occurs for around 25 to 30% of daylight hours in wild populations. In domesticated horses, drowsiness takes up around 8% of the day when horses are indoors and 13 to 14% of the time when outside. The increased drowsiness outside is combined with decreased SWS and REM sleep and is again related to survival and predation. During periods of drowsiness horses may prefer specific places in their stable – especially the corners. The posture is characteristic with the weight evenly on the front legs and one hind leg – the other hind leg is flexed and balancing on the toe. The head is usually above wither height. If startled the flexed leg is often raised (as if preparing to kick) and sometimes stretches backwards.
Like many other prey animals, horses sleep in social groups. Not all sleep at the same time and a sentry usually remains standing while the others lie down to sleep. Changes in the guard don’t always go smoothly, but generally the first sentry will not lie down for at least 10 minutes after the new one stands up. The shape of the paddock or yard affects REM sleep time in groups of horses, and studies have demonstrated that a rectangular area is preferable to a square – even when the total area is the same.
Like us, horses sleep because they have to. Consciousness tires the brain and is not something that can be kept up indefinitely. Sleep deprivation results in changes in the electrical activity in the brain, leading to epileptic seizures in extreme cases. If forced to stay awake, or deprived of REM sleep around the clock, abnormal states develop, starting with irritability, followed by fainting, hallucinations, metabolic collapse and death. And we need REM sleep. The current theory is that during sleep in general and REM sleep in particular, neural irrelevancies are erased and all innate systems are reset – like rebooting a computer.
A period of full, flat-out, lying on their side is necessary for healthy equine sleep, and lack of it can lead to sleep deprivation, which can be caused by many things including environmental stress, noise, extreme temperatures, unfamiliar or unsafe spaces, feeding, fasting, pasture, type of confinement, social hierarchy and grouping, age, feeding program, weather, size of stable and bedding type. Problems lying down also affect the opportunity for REM sleep – painful medical conditions such as arthritis, old fractures, enteroliths or neurologic disease can cause some horses not to lie down because getting up and down is uncomfortable.Your veterinarian may begin pain management to see if the horse’s REM sleep improves with treatment.
Partial collapsing episodes related to REM sleep can occur in horses that, for whatever reason, refuse to lie down. Late pregnant mares that do not lie down can have multiple episodes of knuckling and almost collapsing, making up for sleep loss with lots of lying down after birth. Sleep deprivation may also occur when horses join or are removed from a herd. One study found ponies moved to a new paddock or stable do not sleep well for the following 24 to 48 hours and suffer sleep deprivation. But if a stable or paddock mate was familiar with the surroundings and did lie down for REM sleep, the new pony did likewise.
Excessive sleepiness can occur in such conditions as PPID, narcolepsy (a sleep disorder characterized by excessive daytime sleepiness and abnormal REM sleep) and other illnesses. Even though they are seen lying flat out on their side, horses with narcolepsy lack periods of REM. In humans, dogs and horses, narcolepsy is triggered by emotions such as play and excitement, and it has a genetic component. Although described in all breeds, families with narcolepsy have been identified in dogs, miniature ponies and Lipizzaners. Affected horses may fall in a heap when startled and foals may have frequent bouts of falling sleep and collapse when excited by play time, going into a paddock, eating, nursing, chewing hay and drinking from a dam – linked to risk of drowning! A complete physical, laboratory, and other diagnostic work-up in horses with collapsing episodes is needed. Video monitoring and keeping good records of the sleep behaviour of a suspect narcoleptic horse is important.
Sleep deprivation can be manifested in the horse as excessive daytime sleepiness and collapsing episodes (not to be confused with narcolepsy and cataplexy). Because the process of lying down requires co-ordination of the muscles, tendons, joints and bones, sick horses may try to REM sleep while standing – resulting in partial collapsing. Unexplained abrasions or scars on the front of the fetlocks and knees may be the result of episodes of collapse. Your veterinarian will consider sleep deprivation in the differential diagnoses if collapsing episodes are the primary complaint. Again, long-term video monitoring can be helpful in characterising the episodes and determining if the horse is spending any time recumbent as well as the duration and behaviour (resting quietly or sleeping) of the recumbency.
Commonly associated with sleep deprivation and excessive sleepiness, is poor performance. The need for sufficient time in lateral recumbency is important for domesticated horses living in environments where they’re asked to adapt their innate behaviours to living conditions that only remotely resemble their natural evolutionary environment. We are increasingly recognising how many health problems in horses are related to stress – both their own and that communicated to them by other horses. To avoid unintentionally imposing the added stress of sleep deprivation, we must ensure they have time, conducive conditions, and opportunity for full, flat-out lying on their side. Fortunately, the majority of cases of sleep deprivation can be corrected if the cause is identified.
We don’t need words to have concepts and neither do horses – so the answer is yes, horses do gallop during sleep – but only in their dreams.
Dr Jennifer Stewart BVSc BSc PhD in an equine veterinarian, CEO of Jenquine and a consultant nutritionist in Equine Clinical Nutrition.
All content provided in this article is for general use and information only and does not constitute advice or a veterinary opinion. It is not intended as specific medical advice or opinion and should not be relied on in place of consultation with your equine veterinarian.
