Endogenous zeitgebers

• Exogenous zeitgebers are external environmental factors that reset biological clocks- 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: an interaction of internal and external factors
• Light is a key exogenous zeitgeber that influences the sleep/wake cycle- it can reset the body's main endogenous pacemaker (SCN) and also had an indirect influence on key processes in the body controlling hormone secretion, blood circulation, etc.
• Key study: Campbell and Murphy

  Woke 15 participants at various times and shone a light on the backs of their knees- producing a deviation in the sleep/wake cycle of up to three hours.

  Showed that light is a powerful exogenous zeitgeber directed by skin receptor sites and does not necessarily rely on the eyes to influence the SCN
• Social cues also have an important influence on the sleep/wake cycle
• The sleep/wake cycle is fairly random in human newborns, but most babies are entrained by about six weeks. Schedules imposed by parents are a key influence, including adult-determined mealtimes and bedtimes
• Research also shows adapting to local times for eating and sleeping (not responding to one's own feelings of hunger and fatigue) entrains circadian rhythms and tackles jet lag