Endogenous Pacemakers and Exogenous Zeitgebers

• What are endogenous pacemakers?

  Internal mechanisms that govern biological rhythms
• What factors can alter and affect endogenous pacemakers?

  Environmental factors such as light
• How does the SCN function in relation to biological rhythms?

  It controls other biological rhythms and links to areas of the brain responsible for sleep and arousal
• How does the endogenous pacemaker govern the sleep/wake cycle?

  • SCN receives information about light from the optic nerve
  • SCN sends signals to the pineal gland
  • Increases melatonin production at night to induce sleep
  • Decreases melatonin production as light levels increase
• What are exogenous zeitgebers?

  Environmental events that reset the biological clock of an organism
• What are two types of exogenous zeitgebers?

  Social cues and light
• EP AO3 - Beyond the master clock

  • SCN research may obscure other body clocks
  • Numerous circadian rhythms in many organs and cells in the body
  • These peripheral oscillators are found in the organs including the lungs, pancreas and skin are influenced by the actions of the SCN, but also act independently
  • Damiola et al - demonstrated how changing feeding patterns in mice could alter their circadian rhythms of cells in the liver by up to 12 hours, whilst leaving the rhythm of the SCN unaffected
  • This suggests other complex influences on the sleep/wake cycle.
• EP AO3 - Interactionist system

  • Endogenous pacemakers cannot be studied in isolation
  • Total isolation studies, such as Michael Siffre's cave study are extremely rare
  • Also Siffre made use of artificial light which could have reset his biological clock every time he turned his lamp on
  • In everyday life, pacemakers and zeitgebers interact, and it may make little sense to separate the two for the purpose of research
  • This suggests the more researchers attempt to isolate the influence of internal pacemakers may lower the validity of the research
• EZ AO3 - Environmental observations

  • 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 question the importance of exogenous zeitgebers
  • For instance, the Inuits of the Arctic Circle are said to 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.
• EZ AO3 - Case study evidence (counterpoint)

  • Evidence challenges the role of exogenous zeitgebers
  • Miles et al 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 and light may be needed