Horses sleep or doze in short bursts, most or which are spent standing up.
Research has shown the amount of sleep varies considerably between individuals, but horses actually need as little as two and a half hours of light sleep per day. They also need a couple of hours of deep sleep about once every forty-eight hours, which must be spent lying down.
Once down the rigidity of the spine makes it difficult for a horse to get up. This takes a considerable amount of muscular energy and is often accompanied by a grunt of effort.
Outside horses sleep better in groups because some sleep while others stand guard. This behaviour emanates from the need to make a rapid exodus from predators.
Horses can sleep standing up because the stay apparatus in their legs allows them to relax their muscles and doze without falling down or expending much energy.
Lesson in equestrian culture: How horses sleep standing up
NewsStaff Il Paretaio 2015-11-24