A03

• What are the strengths of Endogenous Pacemakers and Exogenous Zeitgebers

  1. Research support from animal studies for role of SCN
  2. Research support from humans for influence of light
  3. Practical applications for jetlag
• What are the limitations of endogenous pacemakers and exogenous zeitgebers

  1. Influence of exogenous zeitgebers may be overstated
• Strength = research support from animal studies for role of the SCN

  • Morgan (1995) - hamsters = support for suprachiasmatic nucleus
  • Bred hamsters so that they had abnormal circadian rhythms of 20 hours rather than 24 hours
  • SCN neurons from these abnormal hamsters were transplanted into the brains of normal hamsters and SCN neurons from normal hamsters into the brains of abnormal hamsters.
  • The normal hamsters displayed the same abnormal circadian rhythm of 20 hours
  • Rather than maintain their abnormal circadian rhythm the abnormal hamsters then changed to a circadian pattern of 24 hours.
• Strength = research support from humans for influence of light

  • Campbell and Murphy (1998)
  • 15 participants were awoken at various times and a light pad was shone on the back of their knees
  • This produced a deviation in their usual sleep-wake cycle of up to 3 hours in some cases
  • Light is a powerful exogenous zeitgeber that does not always rely on the eyes to exert its influence on the brain
• Strength = practical application for jetlag

  • Burgess et al. (2003) - volunteers participated in one of three treatments before an east-west flight (continuous bright light, intermittent bright light and dim light)
  • Each shifted their sleep-wake cycle back by 1 hour a day over 3 days
  • Those exposed to continuous bright light prior to flight needed less time to readjust to local time on arrival
  • Suggests light exposure prior to a flight would allow travellers to arrive with circadian rhythms partially re-entrained to local time
• Limitation = influence of exogenous zeitgebers may be overstated

  • Miles et al. (1977) - a young man blind from birth, with circadian rhythm of 24.9 hours
  • Despite exposure to social cues, his sleep-wake cycle could not be adjusted and he had to take sedatives at night and stimulants in the morning to keep pace with the 24-hour world
  • The study of individuals who live in arctic regions show normal sleep patterns despite prolonged exposure to light
  • Examples suggest there are occasions where exogenous zeitgebers may have little bearing on our internal rhythm