Tonight nearly every single one of us will fall asleep and each and everyone who does so will dream. It’s an extraordinary world of pleasure and pain but why we dream is one of sciences great mysteries. It’s only now that science is beginning to reveal the bizarre complexities of this secret world. Much of what we thought we knew about dreams now appears to be wrong. This BBC documentary asks the question where do are dreams come from, what do they mean and do they have a purpose?