Horses may not require as much sleep as humans, but quality sleep is still vital for your equine’s overall health and well-being.
Although horses can sleep standing up thanks to their unique stay apparatus, REM sleep is only possible when they are lying down, and their muscles can relax.
Many factors can prevent a horse from getting enough quality sleep and lead to signs of sleep deprivation. Factors include pain, injury, health conditions, loud or bright barn environments, and even social hierarchy.
While equine sleep disorders are still poorly understood, horse owners should be aware of the signs of sleep deprivation in horses and take action to improve their horse’s sleep quality.
This article will review the current science on equine sleep patterns and the consequences, signs, and treatment of sleep deprivation in horses.
Equine Sleep Cycles
Little is known about equine sleep behaviour. However, research suggests that horses experience several different sleep patterns and that they cannot experience the full range of sleep cycles while standing. 
Like humans, horses go through several sleep cycles with different stages. These stages include rapid eye movement (REM) and non-rapid eye movement (NREM) sleep. 
Researchers divide NREM sleep into four stages (N1 – N4) differentiated by brainwave patterns. The stages represent the gradual transition from wakefulness to sleep onset followed by deeper stages of sleep. 
Most horse owners are familiar with the tell-tale signs of N1 sleep: a lowered head, semi-closed eyes, and a droopy bottom lip as the horse becomes drowsy and falls asleep.
Full sleep onset is marked by the transition to the N2 stage, with greater theta wave activity. This is the predominant sleep stage seen in humans and researchers routinely observe this stage in horses. 
Equine Stay Apparatus
Horses can slip into light NREM sleep with little effort and remain standing up. This is due to the unique arrangement of their musculoskeletal system known as the stay apparatus.
The stay apparatus is a collection of ligaments and tendons that work together to keep the horse’s body upright with minimal muscular energy.
It works by locking major joints into place and stabilizing the limbs so the horse won’t lose its balance or fall over. 
Passive Stay Apparatus
Front limbs have a passive stay apparatus that involves the shoulder joint, biceps muscle, elbow joint, and triceps muscle. The biceps muscle prevents shoulder joint flexion while the triceps muscle fixes the elbow joint in extension. 
The Lacertus fibrosus tendon extends from the biceps tendon and attaches below the carpal joint to lock the limb in extension without using the muscles. The flexor tendons and suspensory ligament prevent over-extension of lower limb joints. 
Pelvic Stay Apparatus
Hind limbs also have a stay apparatus to stabilize the stifle, hock, and distal limb during rest. The stifle-locking mechanism consists of the distal patellar ligaments and the collateral ligaments of the stifle. 
Locking the stifle also stabilizes the hock joint through the reciprocal mechanism, which requires the stifle and hock to work in unison. The suspensory ligament and distal limb tendons stabilize the lower joints. 
The pelvic stay apparatus allows the horse to support its body weight without significant exertion. Horses will shift their weight while relaxed and cock one of their hind legs to rest the leg that isn’t bearing weight. 
Although horses can enter light sleep stages while standing up, they cannot enter REM sleep without lying down in a lateral recumbent position. 
Most horses will lie down on their sternum (floor of the chest) during slow-wave sleep and only begin REM sleep once fully relaxed on their sides. 
REM sleep is a paradoxical sleep stage with a mixed frequency of low-amplitude brain waves also observed during wakefulness. This sleep stage involves the inhibition of spinal motor neurons to suppress skeletal muscle tone and reflexes. 
During this phase, the eyelids are closed, muscles relax, and rapid eye movement occurs.  Owners sometimes catch their horses dreaming during this phase and may notice facial twitching, limb contractions, or even vocalization in some horses.
While all stages of sleep are essential, REM sleep is vital for mental, emotional, and physical health.  If horses are unable or unwilling to lie down, a lack of REM sleep can harm their quality of life.
How Much Sleep do Horses Need?
Adult horses typically require between 3 and 6 hours of total sleep and 30 minutes of REM sleep in a day. 
Young foals require up to 12 hours of sleep per day and are even more susceptible to the harmful effects of sleep deprivation. 
Sleep cycles in horses can be as short as 15 minutes.
Horses are polyphasic sleepers, meaning they sleep multiple times throughout the day. However, most slow-wave and REM sleep occurs at night. 
Why Won’t my Horse Lay Down to Sleep?
There are several reasons why horses may not lie down to sleep. Some horses feel uncomfortable in their sleeping environment due to poor bedding, small stall size, or separation from other horses. 
Other potential causes include pain and stress.  If your horse isn’t lying down to sleep, evaluate their environment and schedule a veterinary examination to determine if your horse has a sleep disorder and what may be preventing them from lying down.
Environmental Factors Influencing Sleep
Your horse’s environment can significantly impact their sleep quality.
Horses are prey animals that rely on constant vigilance to survive in the wild. If your horse doesn’t feel secure in their environment, their sleep quality will suffer. 
Living in herds allows individuals to take turns sleeping. If your horse lives alone, they may not feel safe enough to sleep. 
Herd dynamics also contribute to security. Wild horses generally have matriarchal hierarchies, and studies suggest domestic equines lie down more when a mare is on the lookout. 
Noise & Lighting
Horses in new environments often have greater vigilance during an acclimatization period to new stimuli.  Sensory stimuli such as bright light and loud noises can disturb equine sleep.
Light exposure regulates circadian rhythms and the production of sleep hormones. Studies observing domestic horses exposed to artificial light at night report a significant reduction in recumbency. 
Like most mammals, horses also experience a reduction in core body temperature during sleep. 
Owners could benefit from more research on the direct effects of ambient temperature on equine sleep. But anecdotal evidence suggests that over-blanketing may contribute to equine sleep disorders. 
Signs of Sleep Deprivation in Horses
Sleep deprivation is a