How Do Horses Sleep Standing up?

A horse has a "stay apparatus" in its legs that prevents them from
buckling, allowing the horse to stand effortlessly.

Equids -- specifically horses, donkeys and zebras -- are the only animals
that sleep standing up, an evolutionary talent that allows them to doze and
still be ready to flee from predators at a moment’s notice. Mature
horses, for example, frequently rest standing up, aided by what’s known
as the “stay apparatus” of the forelegs and the “check apparatus”
of the hind legs. This anatomical system lets them snooze without falling
down, with little or no muscular effort.

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