Recognizing and Treating Sleep Deprivation in Horses 

Dr. Joe Bertone reviewed features and categories of equine sleep deprivation based on more than 1,000 cases he’s seen since 1998.
Sleeping horse, horse suffering from sleep deprivation
Causes of sleep deprivation in horses include physical discomfort, environmental insecurity, boredom, aggression displacement, night terrors, and Lyme disease. | Getty Images

That horses get all their required sleep while standing is an outdated and incorrect concept. Sleep deprivation can and does occur, albeit rarely, but it is an important concern for the well-being and safety of horses and owners.  

What, exactly, does sleep deprivation look like in horses?  

According to Joe Bertone, DVM, MS, Dipl. ACVIM, Emeritus Professor of Equine Medicine at the Western University of Health Science’s College of Veterinary Medicine, in Pomona, California, horses expressing sleep deprivation behaviors drop their heads low to the ground, begin collapsing, catch themselves before lying down, and repeat.  

Making a clear distinction between sleep deprivation and narcolepsy, he said, “The term narcolepsy is a very specific diagnosis, and no horse has ever met the criteria needed for that diagnosis.” 

Bertone presented information from 876 cases of sleep deprivation syndrome during the National Alliance of Equine Practitioners (NAEP) 2024 Saratoga Equine Practitioners Convention. These were cases he consulted on from 1998 to the present, driving foundational work on clinical sleep deprivation in horses.  

Before presenting that data, however, Bertone reviewed basic features and functions of equine sleep.  

Why do Animals Sleep? 

“There are a number of theories, but really we aren’t sure why horses, or any animals, sleep,” said Bertone.  

The “Reparative Maintenance Theory” partially explains why animals sleep. This theory suggests that the body repairs and restores itself by replenishing energy stores, repairing tissues, and regulating various physiological processes that are depleted while awake. Another theory is the “Cellular Debris Theory,” where the brain doesn’t have the capacity to turn metabolites over fast enough while awake and debris accumulates. Sleep gives an animal the time to clear the brain of leftover byproducts of metabolism.  

“Likely, it’s a combination of these mechanisms and others,” noted Bertone.  

He also presented the “Adaptive Theory” of sleep.   

“This theory, in simplest terms, proposes that we have three purposes in life: eat, avoid being eaten, and have sex,” he said. “So, if an animal is not doing one of those three things, then there isn’t any point in being awake.” 

Phases of Sleep  

The important phases of sleep for horses are diffuse drowsiness (DD) and paradoxical sleep (PS). During DD, the brain waves are large and slow and critical for entering PS, which is the phase when rapid eye movement (REM) occurs. PS is also called the “dream phase.” 

“PS is critical and can only be achieved while a horse is lying down, because the body must be ‘paralyzed’ in this sleep phase,” Bertone emphasized.  

Thus, horses do not fully sleep without lying down.  
 
Prior to lying down, horses quickly rise out of DD to look around and make sure the environment is safe, with their herd mates awake and observant. Specifically, they check that the lead mare is awake and standing. When the lead mare lies down, most of the herd is often standing and acting as sentinels.  

Horses only require about 10 to 15 minutes of recumbency per day, but they can go weeks if needed without lying down and reaching the PS phase.  
 
“Horses need to move, run, avoid being prey, and migrate,” said Bertone. “There are times when they simply aren’t able to lie down. The sleep patterns of all species are molded by evolution.” 

Characterizing Sleep Deprivation 

Horses can only go without PS for about two to four weeks before behavior changes occur. 

If a horse begins to show collapsing behavior, Bertone recommends thinking about what changed in the horse’s environment in the past two to four weeks. Did a herd mate die? Did the stall change? Were horses moved in the barn? Was the environment changed in any way? Bertone has seen something as innocuous as a lightbulb wattage change near a barn cause sleeping issues. 

Based on Bertone’s observations, sleep deprivation falls into three categories. But he has only identified the exact cause of sleep deprivation behavior in about 58% of his cases. 

“Likely there are more causes,” he said.  

The six classifications of sleep deprivation Bertone currently uses are: 

  1. Physical discomfort that stops a horse from lying down. Did the horse sustain an injury two to four weeks ago, or has pain progressed to the point the horse finds it uncomfortable to lie down or get up? Is a mare heavily pregnant and having difficulty lying down or rising? 
  1. Boredom/monotony. If a horse remains standing in the cross-ties for too long, he might get diffuse sleepiness, then begin to collapse.  
  1. Environmental insecurity. If they are uncomfortable in the environment, horses might refuse to lie down. For example, if a horse is bought from a herd situation and is now placed alone or in an uncomfortable herd situation, he might not lie down. “Many geldings and mares need a dominant mare for security,” said Bertone.  
  1. Aggression displacement, when geldings believe they must act as the dominant mare. Bertone shared a story of a gelding that would begin showing signs of DD and collapse when put in cross-ties. He was exhausted. He tried to be dominant over the other geldings 24 hours a day. He spent all his time beating up the other geldings to the point the owner couldn’t turn him out with them. The theory is, when placed in cross-ties, he was now dominated, and the need to lie down became overwhelming. He was ultimately sold and placed with a dominant mare, which solved his problems. He gained weight, and the collapsing behavior and rideability improved. “Then he was great! He laid out flat in the pasture, completely asleep. All he needed was a nasty, grumpy, dominant mare,” said Bertone.  
  1. Night terrors. Similar to people, these horses “bolt upright” and never reach paradoxical sleep.  
  1. Lyme disease, caused by Borrelia spp. in the spinal cord and muscle.  

Treating Sleep Deprivation 

Once you’ve ascertained the cause of sleep deprivation (Bertone provided a concise algorithm to help with this in the conference proceedings), it might be possible to treat or manage the horse. Pregnancy-associated sleep deprivation will be self-resolving, and musculoskeletal or soft-tissue pain can be treated. Veterinarians can prescribe doxycycline for Lyme and possibly trazodone for night terrors. Horses with sleep deprivation due to boredom/monotony simply require frequent stimulation (and expedited grooming sessions) to stop them from going down in the cross-ties.  

Bertone said one important finding in his work is that while most categories of sleep deprivation have no gender predilection, more geldings are classified under aggression displacement and environmental insecurity than mares. 

The prognosis can become poor for horses that go unattended with Sleep Deprivation Behavior for too long. Even if the cause is identified and managed, the behavior persists.  

1,100 and Counting 

Although this presentation was based on 876 cases, Bertone said he has seen closer to 1,100 cases now. He can be reached at Jbertone@westernu.edu or www.EquineSleepDisorders.com to consult on cases.  

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