Rem Sleep: Understanding Horses' Dreams And Sleep Patterns

Horses are polyphasic sleepers, meaning they have multiple periods of sleep throughout the day, with the majority occurring at night. Horses can sleep standing up, but they need to lie down to achieve rapid eye movement (REM) sleep. Horses that do not lie down for environmental reasons or pain suffer from an REM sleep deficiency that negatively affects their welfare and health.

REM sleep is associated with dreaming and muscle relaxation. Horses need to lie down to enter this state because they lose all muscle tone and will collapse if they are standing. Horses need at least 30 minutes of recumbency per day to fulfil their REM sleep needs.

Horses that are sleep-deprived may collapse or fall asleep standing up. They may also experience excessive daytime drowsiness, abrasions on the knees and fetlocks, and impaired athletic performance.

Horses need to lie down to enter REM sleep

REM sleep is associated with dreaming and muscle relaxation. Horses need a minimum of 30 minutes of recumbency per day to achieve 3.5 to 4.5 minutes of REM sleep. Horses that do not lie down due to environmental insecurities or pain suffer from an REM sleep deficiency, which can cause substantial impairment of welfare and health.

Horses can go through slow-wave sleep (SWS) in both standing and recumbent positions. However, the muscle atonia associated with REM sleep requires sternal or lateral recumbency. Horses usually fall asleep shortly after lying down, so recumbency can be used as an inferred measure of sleep.

Horses that are unable to lie down due to unsuitable environmental conditions, social insecurity, or physical complaints can suffer from REM sleep deficiency and excessive drowsiness. Affected horses may transition into REM sleep while standing and partially collapse before suddenly waking up.

Horses suffering from chronic musculoskeletal disease may be hesitant to lie down due to pain or mechanical difficulties during lying down or rising.

Horses need at least 30 minutes of REM sleep a day

Horses are polyphasic sleepers, meaning they have multiple periods of sleep throughout the day, with most sleep occurring at night. Horses can sleep while standing and while lying down. However, horses need at least 30 minutes of rapid eye movement (REM) sleep a day, which can only be achieved while lying down. During REM sleep, a horse's eyes move back and forth rapidly, and the horse may twitch its ears or skin. Horses that do not lie down for environmental reasons or pain suffer from an REM sleep deficiency that negatively affects their welfare and health.

REM sleep is associated with dreaming and muscle relaxation in humans. Horses need a minimum of 30 minutes of recumbency a day to fulfil their REM sleep needs. Horses can rest and achieve certain types of sleep while standing, but the REM phase cannot be entered without recumbency due to the loss of muscle tone during this phase.

Horses that do not lie down may suffer from excessive drowsiness and may transition into REM sleep while standing and partially collapse before suddenly waking up. Horses with inadequate sleep are at a greater risk of illness and injury.

Horses with symptoms of REM sleep deficiency had shorter lying times and reduced times spent moving, indicating a general compromise of their well-being.

Horses can suffer from sleep deprivation

Horses are prey animals, and as such, they only lie down when they feel safe to do so. If they are in an environment with wild animals, such as cougars, they may be too afraid to lie down. Similarly, horses that are kept alone may feel they have to stay awake to protect themselves. Horses that are trailered to shows or kept in noisy, active stables at night may also be too disturbed to lie down and sleep.

Inadequate bedding or a lack of space to lie down comfortably can also prevent horses from getting the rest they need. Horses with arthritis or other painful conditions may be unable or afraid to lie down. Insufficient bedding can also make it uncomfortable for horses to lie down.

Horses that are sleep-deprived may be tired and sleepy during the day, and in the worst cases, they may abruptly fall asleep standing up and collapse if they go into REM sleep. This can lead to abrasions on the knees caused by falling down asleep.

Sleep deprivation can occur over a period of just five to seven days of incomplete REM sleep. Symptoms of sleep deprivation can include excessive daytime drowsiness, abrasions on the knees and fetlocks, reluctance or inability to lie down, and impaired athletic performance.

Horses can suffer from narcolepsy

Narcolepsy is a rare and incurable sleep disorder of the central nervous system (CNS) that affects horses. It is characterised by spontaneous "sleep attacks" at inappropriate times, usually accompanied by an uncontrollable loss of muscle tone (known as cataplexy). The exact cause of narcolepsy in horses is unknown, and there is currently no cure.

Narcolepsy is a rapid eye movement (REM) sleep disorder that results in excessive sleepiness during the day and "attacks" of sleep that are accompanied by rapid eye movements and always triggered by external stimuli. Horses with narcolepsy may exhibit a temporary loss of muscle tone and reflexes (cataplexy). They may also experience a lowering of the head, buckling of the knees, and sudden collapse.

Narcolepsy in horses can be categorised into two groups: onset at birth or within a few weeks of age, and adult onset. It appears to be breed-specific, with familial narcolepsy reported in Lipizzaners, Miniature Horses, Shetland ponies, and Suffolks. However, sporadic cases have also been noted in other breeds. Horses with pituitary pars intermedia dysfunction (PPID) have also been reported to exhibit narcolepsy-like episodes.

Narcolepsy remains poorly understood, and much of what is known about the condition in equines is based on human sleep studies or behavioural observations. One intrusive study carried out at the University of California in 2008 used 24hr electroencephalography (EEG) and video monitoring equipment, which involved placing 16 needle electrodes under the skin of a horse's scalp.

It is believed that a biochemical malfunction within the complex network of neurotransmitters in the brain may be the cause of sleep disorders such as narcolepsy. Studies have suggested that there are at least two components to the condition: excessive daytime drowsiness and cataplexy.

Excessive daytime drowsiness is when normal consciousness suddenly gives way to REM sleep during periods of relative inactivity, such as standing in a stable or at grass. Signs include a progressive lowering of the head, with the forelegs spread and the hindquarters sagging, and the eyes partially closed, possibly exhibiting rapid flicking from side to side. Some pony breeds may sit on their haunches or roll on their sides. This may last a few seconds or several minutes, and the horse will then wake up calmly and carry on as normal.

Cataplexy is specific to narcolepsy and is a potentially more serious sudden loss of muscle tone and subsequent collapse. While many cases appear to occur spontaneously, others seem to be triggered by surges in positive emotion, such as being turned out in the field, being fed, stroked, or groomed vigorously. The horse's legs may buckle suddenly, and it will either wake up or sink totally. It will often collapse forwards violently, causing trauma to the fronts of the knees and/or fetlocks. Spinal reflexes are absent during this stage, while REM may be visible.

Spontaneous recovery is usually quick, with the horse appearing completely calm and normal afterwards. If the horse is made to walk during the buckling stage, the loss of muscle tone will result in a "drunken", uncoordinated action.

Some horses experience pseudo-narcolepsy, where they fall asleep standing up due to fatigue. Horses are amazing creatures that seem robust and durable, but they are quite delicate and sophisticated. It is important to consult a veterinarian if narcolepsy is suspected in a horse.

Horses can suffer from hypersomnia

Hypersomnia can be a primary problem or due to an underlying endocrine or neurologic disease. Horses with hypersomnia may have a complete veterinary workup with video monitoring observations and a continuous electroencephalography (EEG) to diagnose and treat their sleep disturbances.

Horses with hypersomnia will be excessively drowsy during the day and may have abrasions on their knees and fetlocks from collapsing episodes. They may also be reluctant or unable to lie down and have impaired athletic performance.

Hypersomnia can be caused by:

• Sleep deprivation due to travel, change in environment or routine, inadequate bedding or lack of a place to lie down comfortably, or orthopedic or neurologic problems that prevent a horse from lying down.
• Endocrine or neurologic disease.

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