Horses Sleep Standing Up. How Do They Snooze Without Falling Over?

If you have ever passed by a field of horses at night, you may have noticed that they rarely lie down to sleep. Why, then, do these mammals sleep standing up? The answer is a matter of survival: Horses sleep while standing to reconcile their need for sleep with the ever-present danger posed by predators.

Horses Need a Lot of Energy to Get Up

Simply stated, horses are big animals that need time and energy to get off the ground. That being said, a horse that is standing up has a much greater chance of escaping when roused from sleep. According to research, other large prey herbivores capable of sleeping on their feet include buffalos, zebras, elephants, and giraffes.

Horses have specialized anatomical features that allow them to sleep while standing on their hooves. Karen Waite, an equine specialist at Michigan State University, shares that the “stay apparatus” consists of a series of tendons and ligaments, which connect muscle to bone and bone to bone, respectively, and run throughout both the forelegs and hind legs. When a horse relaxes its leg muscles, the ligaments and tendons of the stay apparatus stabilize the shoulder, ankle, and knee joints in the legs by acting as tension bands. This enables the horse to maintain its upright position with minimal muscle tension.

While horses spend the majority of their time sleeping standing up, they do need to lie down for more restorative sleep. Sarah Matlock, a senior instructor of equine behavior at Colorado State University, says that horses spend the majority of their time sleeping standing up, but they do not achieve full REM sleep when standing. During REM (rapid eye movement) sleep in humans, when the eyes move rapidly under closed eyelids, dreaming can happen, so muscles become temporarily paralyzed to prevent us from acting out or visions.

During REM sleep, the brain consolidates and processes new information in preparation for long-term memory. People who lack sufficient REM sleep may experience difficulties with mental concentration and mood regulation, a weakened immune system, and diminished cell growth.

They Need to Lie Down at Some Point

Adult horses can often survive on as little as five hours of sleep per day, with the majority of that time spent upright. While standing horses can accomplish “slow-wave” or deep, dreamless sleep, a well-rested horse requires at least 25 minutes of REM sleep per day, which can only happen when the horse is lying down, according to Matlock.

Because of this, horses must lie down daily. If they don’t, they may suffer from sleep deprivation, which poses a serious health risk. According to the American Association of Equine Practitioners, this may place horses at risk of injury from collapsing.

Matlock also stated that because standing while sleeping is an adaptation to avoid predators, horses must feel comfortable and secure before they will lie down and sleep. In populations of feral horses, if multiple horses lie down for REM sleep at the same time, there will always be at least one of them that remains standing and likely vigilant for potential predators.