Resetting biological clocks is a process known as entrainment. Without external cues, the free running biological clock continues to ‘tick' in a cyclical pattern. Zeitgebers reset the sleep-wake cycle.
Light can reset the SCN and also has an indirect influence on key processes in the body, controlling hormone secretion, blood circulation etc. Campbell and Murphy woke 15 participants at various times and shone a light on the backs of their knees which produced a deviation in the sleep-wake cycle of up to 3 hours. Therefore light doesn't solely rely on the eyes to influence SCN.
The sleep-wake cycle is fairly random in newborns but most babies are entrained by 16 weeks. Schedules imposed by parents are a key influence including meal times & bed times. Research on jet lag shows adapting to local times for eating and sleeping - so not responding to ones own feelings of hunger and fatigue - entrains circadian rhythms & tackles jet lag.
