HOW CAN HORSES SLEEP WHILE STANDING UP?
Horses have a system of tendons and ligaments called the ‘stay apparatus’ which allows the horse to lock their legs so they can relax their muscles and sleep without falling over. This system is a massive advantage for this prey animal. Should a predator come close in the wild the horse is able to run without having to struggle to get up.
In the wild horses take turns for rest and sleep while others in the herd remain on high alert for predators.
There are four stages of sleep in horses: wakefulness, drowsiness, slow wave sleep and REM sleep.
In wakefulness the horse is alert, standing with all limbs, unless moving. During drowsiness horses can rest a “pelvic limb” by bearing the weight of both limbs on one leg. During slow wave sleep horses can stand with their head low if it’s safe and/or comfortable.
REM sleep is similar across all species and seems to occur mostly when the horse is lying down flat on they side, rather than resting on their chest.
In short, horses are able to doze off while standing up due to the hammock-a system of tendons and ligaments that create the ‘stay apparatus’ to lock their legs. However in order to achieve REM sleep horses must be able to lie down.