Judging by the mockingbirds who sit on my chimney .... all night long ... and sing, sing, sing ... my guess is that mockingbirds never sleep. And some nights, neither do I :-)
According to an old folktale, a dying prince would be saved only if the princess who loved him could spin 10,000 spools of thread in a few weeks. The princess couldn't do the job alone. Her three cats helped her, working hard at three tiny spinning wheels. For their efforts, the cats were rewarded with the ability to purr, to make a noise like the whir of a spinning wheel.
Scientist say the cat has the ability to narrow the space in its larynx or voice box. Air passes through the larynx on the way to and from the lungs. By narrowing the space the cat disturbs the smooth flow of air. You hear the resulting rough flow of air as a purr.