Lynne Peeples travels around the world to discover the ways in which our circadian clocks affect our days - and our nights. Speaking to biologists, professional athletes, surgeons, astronomers, software developers and sleep scientists, she reveals the cutting-edge research that is newly available about the powerful sway circadian rhythms have on our bodies and our cultures. References to a circadian clock date back to the ancient Greeks but research has only recently exposed its complex machinery and the astounding breadth of its functions, and only in the last few years have we begun connecting the dots and developing the tools to put all of this information to use. Threats to our body clocks abound: artificial light, air pollution, jet lag, late-night meals and a myriad of other modern insults. But we can also harness our internal rhythms to our advantage - learning the optimal time to take medication, schedule a job interview or eat dinner. The time has come to reckon with the effects of circadian cycles on our minds, bodies and societies.