our endogenous pacemakers are reset each day by cues in the environment (e.g. light cues of sunrise and sunset). this is called entrainment. as demonstrated in Siffre’s cave study, if we did not have exogenous cues our free-running circadian rhythm continues to tick in a cyclical pattern of 25 hours. exogenous zeitgebers and everyday cues throughout the day help to entrain our circadian rhythm within 24 hours. our sleep-wake cycle is determined by an interaction of internal factors (endogenous pacemakers) and external factors (exogenous zeitgebers)