Psychologist Kimberley Wilson and Dr Xand van Tulleken take a journey around the human body, asking what it can tell us about our innate capacity for change. In this episode, they explore serotonin - a chemical involved in everything from sleep to bowel function, appetite to blood clotting. Xand and Kimberley find out why serotonin’s reputation as the “happy chemical” is misleading, explore the interaction between serotonin and dopamine in sex drive, and hear about new approaches to treating depression using psychoactive drugs. Producer: Dan Hardoon Researcher: Emily Finch Executive Producer: Kate Holland A Whistledown production for BBC Radio 4