A circadian rhythm is a natural cycle that repeats roughly every 24 hours and originates inside the organism rather than being imposed by the environment. A clock coordinates biological processes so they happen at the right time of day, which improves an organism's fitness. These rhythms appear in animals, plants, fungi and cyanobacteria, and they seem to have evolved independently in each group. The clock is adjusted by external cues called zeitgebers, the German word for time-givers, including light, temperature and cycles in the body's chemistry. When the clock falls out of step with these cues, the result in humans can be a circadian rhythm sleep disorder.

In 1909 D. W. Griffith made a 14-minute film about a speculator who corners the wheat market and doubles the price of bread. A farmer sows his field while a 'Wheat King' gambles in the Chicago wheat pit; bakers raise the loaf from five cents to ten and the poor line up for bread. Griffith cut back and forth between the farm, the feast and the breadline, a technique called parallel editing, so audiences saw both ends of the economy at once. The film drew on Frank Norris's stories and on a real scheme: months earlier Chicago financier James Patten had cornered wheat, sent bread prices up and made about $5 million, roughly $185 million today.