Find out what scientists know about dreams: what they are and why we have them.

What are dreams and why do we have them? Are they a window into a hidden realm within us? Science is only just beginning to understand. NOVA joins the leading dream researchers and witnesses the extraordinary experiments they use to investigate the world of sleep. From human narcoleptics to sleepwalking cats, from recurrent nightmares to those who can't dream, each sequence contains a vital clue to the question these scientists are pursuing: Why do we dream?