A new season of the science documentary series begins. We've come a long way since the ancient
Egyptians believed that our dreams were messages from the gods. But even Freud's Interpretation of Dreams is old hat. New advances in our understanding of the brain mean scientists now know far more about how dreams are generated and what may cause their hallucinatory nature. But what they still don't know is why we dream. Peter Evans investigates. Producer Alexandra Feachem