exogenous zeitgebers

• external factors that reset the biological clock 


  The German word zeitgeber means ‘time giver’

  Resetting biological clocks is a process known as entrainment.

  Without external cues, the free-running biological clock continue to ‘tick’ in a cyclical pattern. Zeitgebers reset the sleep/wake cycle: an interaction of internal and external factors.
• light

  Light = key exogenous zeitgeber in humans. can reset the bodies main endogenous pacemaker (SCN) & also has an indirect influence on key processes in the body controlling hormone secretion, blood circulation etc.
• research into light as an exogenous zeitgeber

  Campbell and Murphy 1998 demonstrated that light may be detected by skin receptor sites on the body even when the same information is not received in the eyes. They woke 15 participants at various times and shone light on the backs of their knees – producing a deviation in the sleep/wake cycle of up to  3 hours. This suggests light is a powerful exogenous zeitgeber detected by skin receptor sights and does not necessarily rely on the eyes to influence the SCN.
• social cues

  The sleep/wake cycle is fairly random in human newborns, but most babies are entrained by about 6 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 ones own feelings of hunger and fatigue) entrains circadian rhythms and tackles jet lag.
• One limitation is that exogenous zeitgebers do not have the same effect in all environments.

  The experience of people who live in places where there is very little darkness in summer and very little light in winter tell a different story from the usual narrative. For instance, people who live within the artic circle have similar sleep patterns all year round, despite spending 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.
• limitation = evidence challenges the role of exogenous zeitgebers

  Miles et al 1977 recount the study of a young man, blind from birth, who had an abnormal circadian rhythm of 24.9 hours. Despite exposure to social cues, such as regular mealtimes, his sleep/wake cycle could not be adjusted. This suggests that social cues alone are not effective in resetting the biological rhythm.
• strength and limitation

  Evidence suggests that people have poorer quality of sleep as they get older. This may be due to natural changes in the circadian rhythm as we age, which means falling asleep earlier and broken at night (Duffy et al 2015).

  HOWEVER,

  Studies have suggested that exogenous factors may be more responsible for the changes in sleep patterns amongst older people. Hood et al 2004, found that management of insomnia was improved if elderly people were generally more active and had more exposure to natural light.