The Welfare Implications of Equine Sleep
By Linda Greening, Hartpury University
Sleep reportedly regulates a range of internal processes, plays a part in memory consolidation, and is governed by homeostatic and circadian processes. Thus, sleep is considered a critically important part of both mental and physical wellbeing for all mammals, with ‘sleep debts’ occurring when an individual fails to achieve an ‘optimal’ amount of sleep. The horse is no exception. Described as a polyphasic sleeper, the horse on average engages in several short sessions of sleep interspersed with longer periods of wakefulness resulting in a total of 3-5 hours sleep per 24 hours. Most commonly, equine sleep is observed between midnight and dawn, which is considered an evolutionary adaption by reducing the risk of attack at a time when the horse is most vulnerable. In a recent questionnaire-based study of nearly 600 horse owners, 29% reported never considering the amount of sleep their horse achieved, suggesting consideration of equine sleep may be under-prioritised when evaluating equine welfare within normal husbandry practices.
Similar to humans, the horse cycles through Non-Rapid-Eye-Movement (NREM) and Rapid-Eye-Movement (REM) during an episode of sleep, the latter of which is usually achieved when the horse lies down (figure 1 lateral recumbency (REM); figure 2 sternal recumbency (REM), figure 3 sternal recumbency (NREM)). Muscle tone reduction is a defining characteristic of the REM sleep state, thus a recumbent posture is the safest within which to adopt recumbency. As a neophobic species, the domestic horse requires a period of acclimatisation to changes in routine or the environment. Observations of nocturnal behavioural in novel environments highlight lower proportions of recumbent behaviour until the horse feels comfortable or safe enough to lie down. In the weeks that follow, the proportion of recumbency increases. REM sleep is a necessary component of equine welfare; recent studies indicate that horses deprived of this sleep state, often associated with a reduction in lateral recumbency, are more likely to display spontaneous partial or full collapse.
Additional factors known to influence the occurrence of recumbency include social isolation, human presence, environmental noise, and musculoskeletal pain that makes it uncomfortable for a horse to lie down or get up. The occurrence of nocturnal recumbency can be influenced by both the type and depth of bedding material used. Straw is associated with greater duration of recumbent behaviours although deeper beds (both straw and shavings) have also been successful in promoting greater overall recumbency, highlighting the importance of comfort to stabled horses. Leaving lights on overnight and the size of the stable are also influencing factors in the domestic environment.
Thus, existing research offers environmental modifications that can be made to facilitate the occurrence of equine sleep. Meanwhile, current research is looking to increase our understanding of the phenomenon of equine sleep and determine the influence of sleep reduction or disruption on cognitive abilities relative to performance. In addition, whilst the link between sleep deprivation and collapse has been made, little is known about the prevalence of collapse episodes or the factors relating to them. If you wish to find out more about current equine sleep research or take part in future questionnaire-based research in this field, a Facebook group exists which promotes advancing knowledge in the field of equine sleep. You can join the Nocturnal Equine Behaviour group here.
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Greening, et al., (2013) Investigating duration of nocturnal ingestive and sleep behaviors of horses bedded on straw versus shavings. Journal of Veterinary Behavior, 8(2), pp.82-86.
Greening, et al., (2020) The welfare implications of owner perceptions and knowledge of equine sleep. In: UFAW Virtual Animal Welfare Conference, June 2020.
Greening, et al., (2021) The effect of altering routine husbandry factors on sleep duration and memory consolidation in the horse. Applied Animal Behaviour Science, 236, p.105229.
Hartman, N. and Greening, L.M., 2019. A Preliminary Study Investigating the Influence of Auditory Stimulation on the Occurrence of Nocturnal Equine Sleep-Related Behavior in Stabled Horses. Journal of equine veterinary science, 82, p.102782.