Most of us need eight hours of sleep a night. Horses can get by with only half the amount & unlike us, they are able to fall asleep standing up, without falling over.
In the wild, horses are prey to wolves and other animals. Lying down, they are much more vulnerable than they are standing up. So over millions of years, their bodies have developed a way of staying upright even when they are asleep. Although most horses no longer live in the wild, they can still fall asleep as their ancestors used to.
The reason they are able to do this is a unique system of ligaments - the chords which bind bones together. A horse's ligaments act like a sling over its whole body. These can lock its joints into a fixed position, so it can stand upright without any coscious muscular effort while sleeps.
It is a pity that human beings have not developed a way of doing this. It could be very handy for long queues or travelling on crowded trains!
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