One of the most striking features of living organisms, both animals and plants, is the way their physiology and behavior have adapted to follow the fluctuations of daily light and nocturnal darkness. A clock in the brain synchronized to environmental cues generates biological changes that vary over a 24-hour cycle—circadian rhythms (from the Latin words circa and diem, meaning “about” and “a day,” respectively). In this way, the earth’s rotation is reproduced in the dynamics of our neuronal circuits.