endogenous pacemakers and exogenous zeitgebers

• endogenous pacemakers and the sleep wake cycle

  The suprachiasmatic nucleus (SCN) is a tiny bundle of nerve cells in the hypothalamus which helps maintain circadian rhythms (e.g. sleep/wake cycle).

  Nerve fibres from the eye cross at the optic chiasm on their way to the right and left visual areas. The SCN lies just above the optic chiasm and receives information about light from this structure.
• Influence of SCN on the sleep wake cycle studies

  DeCoursey et al. (2000) destroyed SCN connections in the brains of 30 chipmunks which were returned to their natural habitat and observed for 80 days. Their sleep/wake cycle disappeared and many were killed by predators.

  Ralph et al. (1990) bred 'mutant' hamsters with a 20-hour sleep/wake cycle. SCN cells were transplanted from the foetal tissue of these hamsters into the brains of normal hamsters, which then developed cycles of 20 hours.
• pineal gland and melatonin are endogenous mechanisms

  The SCN passes information on day length to the pineal gland which increases production of melatonin during the night.

  Melatonin is a hormone that induces sleep and is inhibited during periods of wakefulness. It has also been suggested as a causal factor in seasonal affective disorder.
• exogenous zeitgebers and the sleep wake cycle .

  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 (interaction of internal and external factors).
• exogenous zeigebers that influence the sleep wake cycle

  Light can reset the body's main endogenous pacemaker (SCN), and also has an indirect influence on key processes in the

  body controlling hormone secretion, blood circulation, etc.

  Campbell and Murphy (1998) 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. Light does not necessarily rely on the eyes to influence the SCN.
• social cues have an important influence on sleep wake cycle (exogenous zeitgeber)


  The sleep/wake cycle is fairly random in human newborns, but most babies are entrained by about 16 weeks.

  Schedules 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 (not responding to one's own feelings of hunger and fatigue) entrains circadian rhythms and tackles jet lag.
• Evaluation of endogenous pacemakers

  One limitation is ethics. Animal studies of the sleep/wake cycle are justified because there are similar mechanisms in all mammals, so generalisations can be made to the human brain. However, a disturbing issue is the ethics involved. Animals were exposed to considerable risk in the Decoursey et al. study and most died as a result. This suggests that studies like these cannot be justified and researchers should find alternative ways of studying endogenous pacemakers.
• evaluation of exogenous pacemakers

  One limitation is endogenous pacemakers cannot be studied in isolation. Only in exceptional circumstances are endogenous pacemakers free running and unaffected by the influence of exogenous zeitgebers. Total isolation studies (e.g. Siffre's cave study) are rare. In everyday life, pacemakers and zeitgebers interact so it may make little sense to separate the two. This suggests the more researchers attempt to isolate the influence of internal pacemakers, the lower the validity of the research.
• Evaluation of exogenous zeitgebers

  One limitation is effects of exogenous zeitgebers differ in different environments. Exogenous zeitgebers do not 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 Innuit Indians of the Arctic Circle have similar sleep patterns all-year round, despite spending around six months in almost total darkness. This suggests the sleep/ wake cycle is primarily controlled by endogenous pacemakers that can override environmental changes in light.
• Evaluation of exogenous zeitgebers

  One limitation is case study evidence undermines effects of exogenous zeitgeber. Miles et al. (1977)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 are not effective in resetting the biological rhythm and the natural body clock is stronger.