ABSTRACT
At the beginning of the twenty-first century we know more than we have ever known about the brain and the states of sleep, dreaming, and hypnosis, but even so we have only begun to scratch the surface. The four are related and intertwined and the more we learn about each the more we are awed by how one touches the other. It is precisely the rapid exchange of new discoveries and information in each of these sciences that propels and accelerates the phenomenal progress we now see in our understanding of each. For thousands of years, roughly until the beginning of the twentieth century, our understanding of sleep was entirely through perspicacious observation and inspired, talented depiction through words and art. The chapter gives evidence that potentially one of the main purposes of sleep is the removal of toxic metabolic byproducts from the brain.
