Horses can sleep while standing up by locking their legs.

The stay apparatus is a complex anatomical mechanism involving the biceps brachii, triceps, and various ligaments in the limbs. In the front legs, the system works by engaging the serratus ventralis muscle and fibrous bands that bypass the heavy lifting of the muscles. In the hind legs, a unique patellar locking mechanism allows the horse to hook its kneecap over a bony ridge on the femur, effectively locking the leg in place.This evolutionary adaptation is critical for prey animals like Equus ferus caballus. By resting in a standing position, a horse avoids the slow and vulnerable process of standing up from the ground, which can take several seconds. Research from Michigan State University indicates that while horses can achieve light sleep standing up, they must lie down for approximately 30 to 60 minutes a day to enter REM sleep. Failure to lie down for several days can lead to sleep deprivation and even physical collapse.Historically, this trait developed as horses transitioned from forest-dwelling creatures to open-grassland grazers millions of years ago. In these exposed environments, the ability to remain upright provided a significant survival advantage against predators like wolves or large cats. Today, domestic horses still utilize this mechanism, often resting one hind leg on its hoof while the other three legs carry the weight through the locked stay apparatus.