Two thousand years ago, Galen, one of the most prominent medical researchers of the ancient world, proposed that while we're awake, our brain's motive force, its juice, would flow out to all the other parts of the body, animating them but leaving the brain all dried up, and he thought that when we sleep, all this moisture that filled the rest of the body would come rushing back, rehydrating the brain and refreshing the mind.
Hai dous mil anos, Galeno, un dos investigadores médicos máis destacados da Antigüidade postulou que mentres estamos espertos, a forza motriz do cerebro, o seu zume, fluía cara a todas  as outras partes do corpo animándoas pero deixando o cerebro seco. Pensaba que cando durmimos toda esta humidade que enchía o resto do corpo volvía rapidamente para hidratar o cerebro e refrescar a mente.