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.
Prije dvije tisuće godina Galen, jedan od najistaknutijih istraživača medicine starog svijeta, tvrdio je da dok smo budni, pokretačka sila našeg mozga, njegov sok, prelijeva se u druge dijelove tijela, oživljavajući ih, ali ujedno ostavljajući mozak potpuno osušenim, i smatrao je da se, kada spavamo, sva ta vlaga koja je prethodno  ispunjavala naše tijelo, vraća natrag, ponovo hidratizirajući mozak i osvježavajući um.