"Why do we dream and why do we repeat dreams?" asks Mila O'Dea, aged 9, from Panama.
Hannah and Adam delve into the science of sleep. From a pioneering experiment on rapid eye movement sleep, to a brand new 'dream signature' found in the brain, they discover how scientists are investigating our hidden dreamworld.
Featuring sociologist Bill Domhoff from the University of California Santa Cruz, sleep psychologist Mark Blagrove from the University of Swansea, and neurologist Francesca Siclari from the University of Lausanne.
Presenters: Adam Rutherford, Hannah Fry Producer: Michelle Martin.