Lying down is necessary for a horse to achieve full rest, despite its evolutionary need to remain ready to flee as a prey animal. While horses possess a unique anatomical feature called the stay apparatus that allows them to doze while standing, this only facilitates light sleep. To complete a full sleep cycle and maintain neurological health, a horse must transition from a standing position to full recumbency. This shift from standing vigilance to lying vulnerability is a complex and deliberate part of equine behavior.
The Physical Process of Lowering
The physical transition from standing to lying down is a coordinated sequence of movements that manages the horse’s substantial weight. The horse begins by shifting its center of gravity backward, lessening the load on the forelimbs. This prepares the forelegs to fold, often initiated by bending one knee and then the other, sinking forward onto the fetlocks.
The hindquarters then lower as the horse draws both hind legs forward, positioning them underneath the body in a crouched stance. The horse uses its forelegs to control the final descent, folding them completely as the brisket and chest make contact with the ground. This process must be smoothly executed to prevent injury, requiring sufficient space and comfortable footing. The reverse process of rising requires a powerful forward thrust, pushing off the forelimbs to regain a standing position rapidly.
Sternal versus Lateral Recumbency
Once lying down, a horse will adopt one of two primary resting postures. The most common is sternal recumbency, where the horse rests upright on its chest and brisket with its legs folded underneath its body. This position facilitates a light stage of rest known as slow-wave sleep (SWS), which can last for several hours.
Sternal recumbency allows the horse to relax most muscles while remaining relatively alert and ready to spring up quickly if danger arises. To achieve the deepest restorative phase, Rapid Eye Movement (REM) sleep, the horse must fully transition to lateral recumbency, lying flat out on its side. This position is required because REM sleep involves complete muscle atonia, or loss of muscle tone, which prevents the horse from maintaining an upright sternal position.
Duration and Frequency of Rest
Horses are polyphasic sleepers, engaging in multiple, short rest periods distributed throughout a 24-hour cycle. An adult horse generally requires between three and five hours of total sleep per day. While the majority of this time is spent standing or in sternal recumbency, a portion must be dedicated to lateral recumbency for deep sleep.
Horses require a minimum of about 30 minutes of recumbency per day to accumulate the necessary short bursts of REM sleep. These REM cycles are typically brief, often lasting only a few minutes at a time, but they are important for nervous system balance and recovery. If a horse is prevented from lying down for several days, it can develop a sleep deficit that may manifest as excessive drowsiness or partial collapse while standing.
When Inability to Rise is a Concern
While lying down is a normal behavior, the inability or reluctance of a horse to rise is a serious indicator requiring immediate veterinary attention. This condition, often referred to as a “down horse,” can stem from various acute or chronic health issues. Common causes include severe pain, such as from colic or laminitis, weakness due to systemic illness, or neurological deficits.
A horse may also be “cast,” meaning it has rolled too close to a wall or fence and lacks the leverage to push itself up. Prolonged recumbency, especially lying flat on one side for more than a few hours, puts tremendous pressure on internal organs and major muscle groups. This excessive pressure can quickly lead to complications like nerve damage, muscle tissue death, and reduced gastrointestinal motility.