exogenous zeitgebers

• resetting biological clocks is a process known as entrainment. Without external cues, the free running biological clock continues to 'tick' in a circadian pattern. Zeitgebers reset the sleep/wake cycle (interaction of internal and external factor)
• light can reset the body's main endogenous pacemaker (SNC), and also has an indirect influence on key processes in the body controlling hormone secretion, blood circulation. Campbell and Murphy woke 15 participants at various time and shone a light on the backs of their knees producing a deviation in the sleep/wake cycle of up to 3 hours. Light doesn't necessarily rely on the eyes to influence the SCN
• exogenous Zeitgebers are external cues that can be used to reset the biological clock
• the sleep/wake cycle is fairly random in human newborns, but most babies are entrained by about 16 weeks. Schendules imposed by parents are a key influence, including adult determined mealtimes and bedtimes. Research on jet lag shows adapting to local times for eating and sleeping entrains circadian rhythms and tackles jet lag.
• One limitation of exogenous zeitgebers is that they differ in different environments. Exogenous zeitgebers don't have the same effect on people who live in places where there is very little darkness in summer and very little light in winter. For instance, the Inuit Indians of the Arctic Circle have similar sleep patterns all year round, despite spending around 6 months in almost total darkness. This suggests the sleep/wake cycle is primarily controlled by endogenous pacemakers that can override environmental changes in light
• One limitation of exogenous zeitgebers is case studies undermines the effects of exogenous cues. Miles et al. reported the case of a man, blind from birth, with an abnormal circadian rhythm of 24.9 hours. Despite exposure to social cues, such as mealtimes, his sleep/wake cycle could not be adjusted. This suggests that social cues alone aren't effective in resetting the biological rhythm and the natural body clock is stronger