SLEEP is vital for good mental and physical health. Despite this, it is a topic that has received relatively little attention in relation to horse care.
While some horses will spend time lying down in the day, most predominantly sleep at night between the hours of 12 midnight and 6am. This makes rest patterns difficult to monitor, which may explain why it has received so little attention.
Horses are polyphasic sleepers, so unlike humans, their sleep is broken up into multiple shorter sessions throughout the day and night. Horses have two phases of sleep, Slow Wave Sleep (SWS) and Rapid Eye Movement (REM) sleep.
SWS is known as ‘rest of the mind’, during this phase the brain cools and there is low metabolic and neural activity, yet, there is some muscle tone which allows the horse to engage their stay apparatus. The stay apparatus is a group of tendons and ligaments that require minimum musculature effort to lock the horse’s major limbs in place. When the stay apparatus is engaged, then the horse can remain standing whilst in the SWS phase of sleep.
REM sleep is known as ‘sleep of the body’ and this is when dreams occur, learning is consolidated and events from the day are processed. During REM sleep, some regions of the brain heat up and there are high levels of brain metabolic and neural activity. In REM sleep, the muscles become completely relaxed. Lack of muscle tone means the stay apparatus cannot be engaged, therefore, to achieve this phase of rest the horse must lie down.
Horses can spend as little as 30 minutes in REM sleep per night, by comparison humans will spend 90 minutes in this phase of sleep. However, studies have shown that although the time in REM is relatively short, it is still essential for the horse to achieve its full sleep cycle. The shortening of the REM phase of sleep is likely related to the fact that horses are a prey species, so are vulnerable to predators when lying down. The fact that this phase of sleep still exists in the horse, despite pressure from predators, hints at how important REM sleep is for normal functioning. Research has shown that a normal, healthy horse that is comfortable in their environment will spend some time lying down every night.
As in humans, there is a huge amount of variation between individuals in how much sleep is required for normal functioning. Additionally, time spent sleeping can be influenced by internal and external factors, including age, nutrition, exercise, stress, weather/ season, social grouping and housing. This makes classifying ‘normal’ time spent sleeping difficult.
Studies on feral or free-living horses have shown that they will spend approximately 5-11% (one to three hours) of their daily time budget lying down. Domestic horses have shown a greater variation, spending on average 3-21% (45 minutes to five hours) of their time budget lying down. Some individuals in the domestic horse studies failed to lie down at all during the observation period. In domestic horses, there are huge differences in management systems, which could account for some of this variation (Chapline and Gretgrix, 2010).
Achieving a full sleep cycle is a biological need for all mammals and we can be sure that horses that spend no time lying down in REM sleep are likely to suffer physiological and psychological consequences. At the other end of the spectrum, the horses that spent a lot of time lying down also showed a range of stress related behaviours (Heleski et al., 2002). More research is required to determine exactly how much sleep is too much, but it is worth noting that both too much and too little time spent by the horse lying down sleeping are indicators of poor welfare.
What can we do to ensure our horses are getting enough rest?
If stabled, ensure the stable is big enough for your horse to get up and down comfortably. Your horse should be able to lie on their side without touching any of the walls in their stable. Studies have shown that most horses prefer straw as their ideal bedding, but this varies by individual. Whatever bedding type you choose, ensure it is deep enough to provide sufficient cushioning of the joints. If a deep bed isn’t an option, then include rubber mats with bedding on top. For horses that are outside, ensure they have access to a dry comfortable place to lie down, a run in shelter with bedding is ideal, but even cover under trees is enough for most horses.
Where possible, monitor whether your horse is lying down – look for signs of bedding or dirt on their coat or rug and depressions in the ground. If possible, monitor CCTV footage or use a device that can be attached to the rug, which will automatically monitor time spent resting. If you suspect your horse isn’t lying down most nights or you feel they are lying down excessively, then have them checked by a veterinarian as soon as possible.
References
Chapline and Gretgrix, 2010. Effect of housing conditions on activity and lying behaviour of horses. Animal, 4(5), 792-795.
Heleski et al., 2002. Influence of housing on weanling horse behavior and subsequent welfare. App.l Anim. Behav. Sci., 78(2-4), 291-302.
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