The stuff of dreams -- not just the content, but the circuitry and neurological coding, too -- is the subject of this penetrating "Nova" inquiry into the most mystifying of human activities. Researchers are taking a variety of investigative tacks, from "eavesdropping" on sleeping rats to methodical studies of thousands of dreams, in an effort to fill in the blanks of understanding. Their findings give tantalizing glimpses of the mind at rest.