summary In average, people spend a third of their life doing nothing but sleeping. It is an instinct that everyone knows how to do and it is sort of daily routine to all of us. We need a good night's sleep; however, do any of us really understand what it is all about? Two thousand years ago, Galen stated that our brain's motive force, which is called juice, would flow out to the other parts of the body, leaving the brain all dried up while we are awake, and he thought that when we sleep, all this moisture that filled the rest of the body would come rushing back, refresh the brain and the mind. Thanks to the modern technology, we have a chance to unveil the mystery of how human bodies work. The first problem that every organ must solve is a continuous supply of nutrients to fuel all those cells of the body. Now, just as every cell requires nutrients to fuel it, every cell also produces waste...