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Animal sleep

• Sleep is so essential to animals that Nature has made special arrangements to enable it to happen. Horses’ tendons are specially adapted to allow them to sleep standing up; similar adaptations allow South American sloths to sleep upside down, and migratory birds to sleep on the wing. Dolphin brains are so constructed to enable them to swim continuously while breathing (half their forebrain goes to sleep while the other half remains awake).

• The amount of sleep animals need varies according to their size. Elephants sleep for four hours, for instance; rats for 14.