Entrainment, within the study of chronobiology, occurs when rhythmic physiological or behavioral events match their period and phase to that of an environmental oscillation. A common example is the entrainment of circadian rhythms to the daily light–dark cycle, which ultimately is determined by the Earth's rotation. The term entrainment is justified because the biological rhythms are endogenous: They persist when the organism is isolated from periodic environmental cues. Of the several possible cues, called zeitgebers (German for 'time-givers', 'synchronizers'), which can contribute to entrainment, bright light is by far the most effective. Exercise may also play a role.