Like humans, sleep is essential for the wellbeing of a horse. However, sleeping patterns and characteristics of horses are unique. Horses are polyphasic sleepers which means they have multiple periods of sleep throughout the day, with the majority occurring at night. Sleep patterns are based on the horse’s environment, social hierarchy, age, feeding, and familiarity with surroundings. One special characteristic of horses is that they can sleep standing up!
How Do Horses Sleep?
Four stages of vigilance have been documented in the horse: wakefulness, drowsiness, slow wave sleep (SWS) and rapid eye movement (REM) sleep. In SWS, a horse’s brain waves are slow and synchronized. During this time, the brain is not functioning at an active level. SWS may occur while the horse is standing or in sternal recumbency (lying on the chest with legs folded underneath).
In REM sleep, brain waves are rapid and irregular, like those of an awakened state. As the name implies, during REM sleep, a horse’s eyes move back and forth rapidly. As well as the eye movement, your horse may also twitch his ears or skin, blink, flare his nostrils, or even paddle his legs. REM sleep occurs while the horse is lying on his side in lateral recumbency. During REM sleep all the muscles completely relax and lose their tone.
On average, most horses spend a combined total of 5-7 hours a day sleeping. It is generally accepted that horses spend approximately 15% of their total sleep time in REM sleep. While some horses get 2-3 hours of REM sleep per day, all horses need at least 30 minutes.
There is no definitive amount of time that horses can lay in lateral recumbency, but they can’t stay there for too long. The horse’s weight alone applies pressure to areas of the body which restricts blood flow to vital organs and limbs. The lungs are also compressed, which can lead to abnormal breathing patterns. The pressure can also affect nerves, rendering the horse’s limbs temporarily paretic (muscle weakness caused by nerve damage). When horses try to get up, they have difficulty standing on all four limbs, which can lead to secondary trauma.
How Long Do Horses Sleep Standing Up?
Most of a horse’s sleep is done in the standing position and is considered SWS, as described above. Total sleep time is usually comprised of cycles of sleep interrupted by periods of wakefulness. A special anatomical feature of horses called the stay apparatus allows horses to sleep standing up. A stay apparatus is a group of tendons and ligaments that work together so that the horse can remain standing with little muscular effort. This is a great advantage for a prey animal, like a horse, so that in the event of an emergency they can quickly awaken and easily flee. Another protective tactic horses use while sleeping is to sleep in groups. They will rotate sentries—horses that will stay standing and alert—while the others rest.
Can Horses Have Sleeping Disorders?
Sleep disorders in horses are poorly understood. Research groups are continuing to study sleep disorders in horses including sleep deprivation, narcolepsy, and hypersomnia.
Sleep deprivation is caused by a lack of sleep, which can occur during travel, change in environment or routine (excessive noise, unfamiliar conditions, inadequate bedding), and orthopedic or neurologic problems that prevent a horse from lying down. Sleep deprivation can occur in as little as 5-7 days of incomplete REM sleep.
Narcolepsy is a sleep disorder triggered by strong emotions and activity. It is characterized by excessive daytime sleepiness and abnormal REM manifestations. Narcoleptic horses may have sudden loss of muscle tone (cataplexy) and sudden onset of sleep.
Hypersomnia is a condition of excess sleep, however this sleep is not restorative and usually doesn’t include REM sleep, so in turn leads to more sleepiness. It can be a primary problem or due to an underlying endocrine or neurologic disease.
Risks of inadequate sleep in horses include:
Common symptoms of horses getting inadequate sleep include excessive daytime drowsiness, abrasions on the knees and fetlocks (from collapsing episodes), reluctance or inability to lie down, and impaired athletic performance. A complete veterinary work-up with video monitoring observations and a continuous electroencephalography (EEG) is recommended to diagnose and treat a horse’s sleep disturbances.