Computer graphics contour map representing a mathematical function which describes the resetting of a circadian (24-hour) body clock by timed exposure to light. Here, the subject is eclosion in a population of Drosophila, the fruit fly. Eclosion is the event when the adult fly emerges from its pupal case. Embryonic fruit flies do not automatically have functioning clocks; flies raised from conception in constant darkness at constant temperature show no preference of a particular hour to emerge. However, a single flash of light may start all the embryonic clocks in the population, with adults subsequently emerging in bursts every 24 hours.