Horses do lie down, but their resting habits are distinct from many other mammals due to their evolutionary history as prey animals. They are uniquely adapted to rest in multiple positions, allowing them to balance the need for sleep with the constant requirement to be ready for immediate flight.
Standing Rest and Sternal Recumbency
For the majority of their rest, horses utilize a standing position, which is a state of light sleep often referred to as a “doze.” They can achieve this resting state with minimal muscular effort thanks to a specialized anatomical feature called the stay apparatus, allowing them to quickly awaken and flee from a perceived threat. When they choose to lie down, they often adopt sternal recumbency, resting on their chest with their legs folded underneath them. This position is common, accounting for approximately 80% of the time an adult horse spends lying down. The sternal position still keeps their head and neck relatively upright and allows for a rapid return to their feet if necessary.
The Physiological Need for Deep Sleep
Despite their ability to rest standing or sternally, horses must lie down completely to achieve the deepest stage of sleep, known as Rapid Eye Movement (REM) sleep. During REM sleep, the body experiences a near-total loss of muscle tone, or muscle atonia, which is incompatible with standing or the partially upright sternal position. They enter lateral recumbency, lying fully flat on their side with their limbs extended, to safely experience this phase. REM sleep is necessary for both mental and physical restoration, and adult horses require a minimum of 30 to 45 minutes of it per day, typically accumulated in short bursts.
If a horse is prevented from lying down due to pain, discomfort, or an unsafe environment, it can become sleep-deprived. Severe sleep deprivation can lead to the horse involuntarily collapsing into REM sleep while standing, which can cause injury as they fall.
Evolutionary Reasons for Standing
Their considerable body size makes rising from a lying position a slower and more difficult process compared to smaller animals, making a quick escape challenging. The ability to rest while remaining upright evolved as a primary survival mechanism. This mechanism is enabled by the stay apparatus, a complex system of tendons, ligaments, and passive structures in all four legs that essentially locks the major joints. In the hindlimb, for example, the patella (kneecap) can be locked over a bony ridge on the femur, stabilizing the stifle and hock joints without continuous muscle contraction. This passive joint stabilization dramatically conserves energy and allows the horse to remain standing for hours with minimal muscular fatigue.