A good night’s sleep is vital for the health and wellbeing of all mammals, including horses. Sleep aids recuperation after exertion and it is essential that nothing inhibits this.
A study of behaviour in a group of treadmill-exercised horses found that they spent twice as much time lying down at night as the group that did no exercise.
Horses don’t need as much sleep as we do — around three hours within every 24 hours, with another two to three hours spent standing in a drowsy state. Unlike us, they tend to sleep in lots of short bouts. As the horse is a prey animal, these short stints — from which a horse can be roused quickly — are probably an adaptation to help him survive attack at his most vulnerable time.
Stabled horses sleep mostly at night, usually between midnight and 4am, principally because there is too much disturbance during the day. Horses in groups at grass often sleep during the day, typically with one or more horses “standing guard” for predators. The sentinel horses are often the highest-ranking (most dominant) in the group.
It has been shown that horses kept in solitary confinement may sleep less, as the “buddy system” in the herd situation, where one horse watches for predators while the others sleep, is absent.
Allowing visual and tactile contact between adjacent horses might provide the necessary reassurance to ensure the necessary amount of sleep — after all, this companionship has been shown to reduce weaving by 85%.
Stages of sleep
There are three phases of equine sleep:
Drowsiness: the horse is relaxed, with his head and neck drooping slightly below withers height. His eyelids, ears and lower lip are relaxed and he will often stand with one hindleg resting. The “stay apparatus” is an anatomical adaptation, which locks the fore and hindlegs in extension without conscious muscular effort, allowing the horse to remain standing up.
Slow wave sleep: this is true sleep, sometimes called “the sleep of the mind”. The brainwaves, when monitored during a test called an electroencephalogram (EEG), are slow and regular, indicating the brain is not functioning at its most active level. Horses can stand during this phase using the stay apparatus to support themselves, but more commonly they lie in a sitting-up position called sternal recumbency.
REM or paradoxical sleep: the brain is almost as active as when awake, but, in reality, this sleep is deeper than slow wave sleep. It is also called “the sleep of the body”, as there is complete muscular relaxation. The horse can enter REM sleep only when lying down, either in sternal recumbency (often propped against the side of the stable) or, more commonly, lying on his side.
Unquestionably, REM sleep is the most important stage in sleep. Although not fully understood, sufficient REM sleep is important for physical and mental wellbeing in all animals. Horses that never lie down will always be sleep-deprived.
It has been estimated that domestic horses engage in around two hours of slow wave sleep in four or five sleep periods, with waking or REM sleep occurring randomly in between. Typically, the horse spends a total of around 45 minutes in REM sleep in bouts of around five minutes each. From time to time, he will stand, move around the box, nibble at his hay, doze for a while and then lie back down for another sleep.
In recent years, there has been a profound change in the husbandry of horses in the UK.
Rather than the traditional deep straw bed covering most of the stable floor, many horses are now kept on rubber mats with minimal bedding, often covering a relatively small part of the floor area. It has been shown that stable size (and hence the size of the bedded area) significantly affects lying down time — the larger the bed, the longer the horse spends lying down. It follows that this husbandry change may have affected time spent lying down, lying position and sleeping behaviour in ways we have not yet grasped — and maybe have been underestimated.
Sleep-deprived human athletes make more mental errors and have slower reaction times, increased injury rates, a less effective immune system and reduced athletic performance. Little or no research has been focused on the effect of inadequate sleep on equine performance, although it is reasonable to infer from human studies that there may be an effect.
Deprivation of REM sleep in humans by awakening the subject at the onset of each REM phase led to psychological disturbances, such as anxiety, difficulty in concentrating and irritability.
Although extrapolation to horses is pure conjecture, it should be considered.
What about bedding material? Studies have found that horses bedded on straw lie on their sides for three times as long as those on shavings, and that bedding depth also influences lying time.
Research has also shown that, given free choice, horses will select straw to lie on rather than shavings, and horses recovering from anaesthesia on straw tend to have smoother recoveries that when recovering on a soft rubber mat.
All of these factors could affect the amount of slow wave and REM sleep that horses obtain when stabled with different bedding systems.
Sleep deprivation syndrome — often erroneously called narcolepsy, which is a different and rare condition in the horse — describes a phenomenon where horses drift into paradoxical sleep while standing up. The horse briefly crumples forward onto his forelimbs, then instantly wakes up.
Recurring episodes can cause telltale calluses to form on the front of the fetlocks. These episodes may be triggered by being groomed or tacked up.
These horses are often REM sleep-deprived because they don’t lie down at night. The causes of this reluctance to lie include limb pain, insecurity, a lack of bedding, insufficient lying space or because they are the dominant horse in a herd but have been isolated from the group by stabling.
The quantity and quality of sleep enjoyed by their horses is a complete mystery to most owners, typically because our principal focus is on training and working horses during their — and our — waking hours. What happens during the night, when we are not there, might be more significant than we think.
Ref Horse & Hound; 19 October 2017