Sleep and Sleep Disorders in Horses, AAEP 2008

Unlike humans, adult horses average only about three to five hours of sleep per day, with sleep events occurring intermittently throughout the day and night, with most occurring at night. Lack of sleep or a sleep disorder has the potential to impact a horse's physical activity, attitude, and quality of life, according to Monica Aleman, MVZ, PhD, Dipl. ACVIM, who discussed sleep phenomena in horses at the 2008 Convention of the American Association of Equine Practitioners (AAEP), held Dec. 6-10 in San Diego, Calif.

In humans, sleep is classified into several stages:

  • Stage 1, the wake stage of drowsiness;
  • Stage 2, light sleep;
  • Stage 3 and 4, slow wave or delta sleep; and
  • REM (rapid eye movement) sleep, during which dreams might occur.

Humans average four to five cycles of non-REM (Stages 1 to 4) and REM during a full night's sleep.

Researchers studying sleep disorders in horses rely on stall confinement and accustoming the horse to electrode inputs to measure readings of an encephalogram of the brain (EEG) and electromyogram of the muscles (EMG). Because horses maintain a vague state of vigilance, it is difficult to measure the transition from wakefulness to drowsiness. Usually, the horse's stance is with full weight on both front legs and a rear leg while the other rear leg is cocked, or "primed," to kick if necessary.

In horses, sleep Stages 2 to 4 are referred to as slow wave sleep; these typically take place either with the horse lying down in sternal recumbency (on his chest and belly, rather than on his side) or standing as in the drowsy stance but with his head dropped lower than the height of the withers. A horse might remain standing if he is uncomfortable with the security of the environment or due to musculoskeletal problems.

For horses only 15% (30 minutes per day) of total sleep is REM-stage. Usually, a horse will lie flat out on the ground, but he can also experience REM sleep while in sternal recumbency. If horses go into REM sleep while standing, they will have partial collapsing episodes due to relaxed muscular tone. A horse in REM sleep experiences rapid eye movements and relaxed neck muscles. Some REM episodes include leg movements, twitching muscles and/or ears, blinking, and nostril flares.

Sleep deprivation occurs in horses and might be evident as excessive sleepiness during the daytime or collapsing episodes unrelated to narcolepsy or cataplexy (excessive sleepiness with sudden loss of muscle tone triggered by strong emotions).

Hypersomnia describes an excessive amount of sleepiness. An affected horse also might display poor performance. This could be related to sleep deprivation, endocrine disease (such as equine Cushing's), neurologic disease (encephalitis, brain trauma, or EPM), or causes that are not yet identified.

Narcolepsy is also characterized by excessive daytime sleepiness and abnormal REM sleep accompanied by cataplexy, hallucinations, or sleep paralysis. Cataplexy is diagnostic for narcolepsy, but a horse with narcolepsy is not necessarily cataplectic. A familial relationship of narcolepsy has been identified in Miniature Horses. Sleep deprivation is often confused with narcolepsy in the horse, yet narcolepsy is associated with disorder of a specific neuropeptide (hypocretin) that regulates sleep and wakefulness.

Chronic, recurrent abrasions over the front of the fetlocks, and possibly the front knees, are indicative of episodic collapsing behavior. Other situations might also deprive a horse of sleep, including environmental stresses (noise, extreme temperature, unsafe environment, or inadequate bedding), issues with herd dynamics, pain-related problems or late-stage pregnancy that preclude ease of lying down, traveling and competition, and hospitalization. Video monitoring is helpful when assessing sleep behavior.

About the Author

Nancy S. Loving, DVM

Nancy S. Loving, DVM, owns Loving Equine Clinic in Boulder, Colorado, and has a special interest in managing the care of sport horses. Her book, All Horse Systems Go, is a comprehensive veterinary care and conditioning resource in full color that covers all facets of horse care. She has also authored the books Go the Distance as a resource for endurance horse owners, Conformation and Performance, and First Aid for Horse and Rider in addition to many veterinary articles for both horse owner and professional audiences.

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