Light and darkness shape our perception of the world. This is true in a literal sense, but also metaphorically: in theology, philosophy, literature and the arts the light of day signifies life, safety, knowledge and all that is good, while the darkness of the night suggests death, danger, ignorance and evil. A closer inspection, however, reveals that things are not quite so clear cut and that light and darkness cannot be understood as simple binary opposites. On a biological level, for example, daylight and darkness are inseparable factors in the calibration of our circadian rhythms, and a...