If you’ve ever walked past a field of horses at night, you’ve probably noticed that they rarely sleep. So why do these mammals sleep standing up? The answer is survival: horses sleep standing up to balance their need for sleep and the constant threat of predators.

Horses are a long lineage of carnivores in the Equine family, and sleeping on their hooves is an adaptation to predators. “Standing gives them a real jump on predators and a better chance of escaping than lying down,” said Karen Waite, an equine specialist at Michigan State University.

Simply put, horses are large animals and take time and energy to move. Thus, when a standing horse is awakened from sleep, it escapes much more easily. The same is reported to be true of other large herbivores that can sleep on their feet, such as zebras, bison, elephants, and giraffes.

Horses have special anatomical features that allow them to stay on their hooves and catch some. According to Waite, these features, known as the “suspension apparatus,” include a series of tendons and ligaments (the soft tissues that connect muscle to bone and bone to bone, respectively) that run through the forelegs and hind legs. When the horse relaxes the leg muscles, the ligaments and tendons of the support apparatus act as tension bands that stabilize the shoulder, knee and ankle joints of the legs. This allows the horse to stand without straining its muscles.

But while horses spend most of their sleep standing, they must lie down to sleep again. “Horses spend most of their sleep time standing, but they don’t fully reach REM sleep while standing,” said Sarah Matlock, senior lecturer in equine behavior at Colorado State University. During rapid eye movement (REM) in humans – when a sleeping person’s eyes move rapidly under their closed eyelids – we can dream and the muscles are temporarily paralyzed, so we are unable to recreate our dreams. During REM sleep, the brain also assembles and processes new information for long-term memories. People who do not get enough REM sleep may experience problems with mental concentration and mood regulation, a weakened immune system, and decreased cell growth.

Adult horses can usually only sleep 5 hours a day, and most of it standing up. Matlock said that while standing horses can achieve “slow wave” sleep, or deep, dreamless sleep, a properly rested horse requires at least 25 minutes of REM sleep per day, which can only happen in the recumbent position.

For this reason, horses must lie down every day. Otherwise, they may experience sleep deprivation, which can pose a serious risk to the horse’s health. For example, the risk of injury from a fall, according to the American Association of Equine Practitioners. “Horses that don’t get enough REM sleep may be misdiagnosed as narcolepsy,” Matlock said. Said. “For example, they might fall while you’re driving them.”

Matlock said that because standing up while sleeping is an adaptation to avoid predators, horses need to feel comfortable and safe before they are willing to lie down and sleep. In wild horse populations, if several horses lie down for a short nap, there will always be at least one horse standing up to watch for potential predators.

“If they don’t feel safe around, or if they don’t have other horses with them or are isolated, they probably won’t feel safe enough to go to bed,” Matlock said.