 Of the several possible cues, called "zeitgebers" (German for 'time-givers', 'synchronizers', 'external timekeepers'), which can contribute to entrainment, light has the largest impact.Units of circadian time (CT) are used to describe entrainment to refer to the relationship between the rhythm and the light signal/pulse.The activity/rest (sleep) cycle in animals is one of the circadian rhythms that normally are entrained by environmental cues.