Monica Aleman Sleep is essential in horses. Lack of sleep and sleep disorders can compromise horses’ health, performance, and quality of life. Limited information and lack of understanding of normal sleep in the horse can result in erroneous interpretation of normal and abnormal sleep, inaccurate diagnosis of presumed sleep disorders, and management failure. Studies of sleep in the horse have relied mainly on behavioral observations, which is the first essential step in the evaluation of sleep. In addition to observation, the use of simultaneous electroencephalography has redefined sleep medicine in humans through the determination of electroencephalogram (EEG) characteristics of sleep and its various stages. This is important because the definition of various sleep disorders in humans depends on identifying alterations in the sequence, duration, and pattern of stages of sleep. Sleep staging is based on specific features recorded in the EEG, electrooculogram (EOG), electromyogram (EMG), electrocardiogram (ECG), and respiratory patterns. Collectively, these studies and their interpretation are known as polysomnography. Polysomnography is an emerging science in veterinary medicine. Horses sleep an average of 3 to 4 hours per day and have multiple phases of rest and sleep (i.e., are polyphasic sleepers) throughout any given 24-hour period. Most sleep in horses takes place at night. Approximately six phases of rest and sleep during the nighttime have been seen in horses in observational studies. Duration of lateral recumbency during the night has been reported to be 2% to 9% and 5% to 15% in wild and stabled horses, respectively. In the wild, horses take turns for rest and sleep, with a few horses remaining on alert while others rest or sleep. Horses on alert stand on higher areas where possible predators can be seen more easily. Foals, especially neonatal foals, spend more time sleeping than adults. Their periods of rest and sleep are higher in number and frequency and are of longer duration than those of adults. The need for rest and sleep in the foal decreases as it matures. Observational studies involving wild horses and those housed in pastures have revealed that a shift in the amount of sleep substantially declines in foals by 3 months of age. Differences in breed and sex have been reported, with draft breeds resting more than light breeds, and fillies resting more than colts. Many factors can influence sleep in horses, including the environment, degree of safety, companions, hierarchy, physiologic state and age, exercise, diet, and disease (Table 93-1). Horses are herd animals by nature, and through domestication humans have likely altered sleep patterns in horses. Sleep staging in horses is not nearly as sophisticated as it is in humans because of the difficulty of performing an EEG in the natural setting. However, the recent introduction of telemetric EEG units in veterinary medicine has opened a major opportunity for studying sleep in horses. Telemetric EEG enables recording of EEG activity from a distance and does not interfere with the horses’ daily activities, although there are limits on the distance over which recording is possible, depending on specific EEG units. Maintaining EEG electrodes also presents a challenge, but surface electrodes can be held in place with collodion for long periods of recording. Additionally, muscle and movement artifacts can seriously compromise the evaluation and interpretation of EEG. However, obtaining an interpretable EEG in the alert horse is possible. In humans, the stages of wakefulness and sleep include awake with eyes open; awake with eyes closed; non–rapid eye movement (non-REM) sleep (stage 1—drowsiness, stage 2—light sleep, stages 3 and 4—slow-wave sleep or delta sleep); and REM sleep. In horses, four states of vigilance can be determined through observation and simultaneous video and EEG recordings. The electrophysiologic evaluation includes EEG, EMG, EOG, ECG, and respiratory monitoring for defining the stages of sleep. The stages include wakefulness, drowsiness, slow-wave sleep, and REM sleep. In wakefulness, the horse is bright and alert, and is standing and bearing weight on all limbs unless moving; movement artifacts (of the eyes, ears, jaw, and head) and EMG activity are common; heart and respiratory rates are within reference values. The transition from wakefulness with eyes closed to drowsiness in humans is determined by the transition from alpha rhythm (8 to 13 Hz), observed during wakefulness, to theta (4 to <8 Hz) and delta activity (<4 Hz). However, the horse lacks a true alpha rhythm, although some background activity falls into the alpha-frequency range. This makes the definition of transition from wakefulness to drowsiness difficult if interpreted with EEG alone. Intermittent 4-Hz activity, along with observations during this stage of sleep, reveals that horses adopt a stance characterized by weight bearing on both thoracic limbs and one pelvic limb, with the other pelvic limb primed (possibly in readiness to kick a predator). Occasional vertex sharp waves (V waves) and transient benign variants that may appear epileptiform have been observed in healthy horses. These are similar to those observed in healthy humans during this stage of sleep. These benign variants also have been observed in sleep-deprived horses. In horses, the equivalent of stages 2 to 4 in humans can be collectively referred to as slow-wave sleep. During this state, other normal transient events—sleep spindles and K-complexes—have been recorded in horses, and this stage is the equivalent to light sleep in humans. The gradual appearance of high-amplitude delta activity is similar between species, signifying a deeper plane of sleep (human slow-wave sleep [stages 3 and 4]). The horse can enter this stage of sleep standing with the head held low or while in sternal recumbency if it feels safe and comfortable. Many horses in this stage of sleep develop a second-degree atrioventricular block that is not present during other states of vigilance. Rapid eye movement sleep is similar among species: low-voltage and mixed-frequency EMG activity is present with episodic fast eye movements. Occasionally, there may even be almost no EMG activity. Horses appear to spend about 15% of their total sleep time in this state. Horses can have REM sleep while in sternal recumbency, while supporting their head on the ground or on an object, or while in lateral recumbency if they feel safe and comfortable. During REM sleep there is loss of muscle tone. However, in addition to rapid eye movements, twitching, blinking, flaring nostrils, and even limb stretching may be observed. These movements have been wrongly interpreted as seizures. Brief standing REM sleep has been documented, but the accompanying loss of muscle tone precludes extended periods and will result in collapse. Lack of REM sleep results in sleep deprivation.
Sleep Disorders
Sleep
Stages of Sleep
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