Evolutionary biologist Ella Al Shamahi and a team of sleep and circadian experts test what happens when the body clock loses every external cue. Former Royal Marine commando Aldo Kane volunteers to spend ten days alone inside a dark underground nuclear bunker, cut off from sunlight, clocks, and human contact, while researchers monitor his sleep, mood, and hormone levels for signs of his internal rhythm drifting free. Interviews with circadian scientists explain the molecular machinery behind the body clock, including the genetic differences that make some people early birds and others night owls, and the film uses Kane's isolation as a live experiment to show how disconnected the body becomes without light and routine. It also looks at the real-world costs of ignoring that clock, from shift work to jet lag, drawing on research linking disrupted rhythms to long-term health problems. Produced for the BBC's Horizon strand, the film pairs Kane's bunker footage with lab demonstrations and expert commentary to build a clear, evidence-based picture of a system most people never think about.