Endogenous Pacemakers & Exogenous Zeitgebers

• What are endogenous pacemakers?

  Internal body clocks regulating biological rhythms
• What is the SCN?

  An endogenous pacemaker governing the Sleep/Wake Cycle
• How does the SCN receive information about light?

  Directly from the optic chiasm
• Where is the SCN located?

  Just above the optic chiasm
• What can reset the SCN?

  Exogenous zeitgebers
• What does the SCN communicate to the pineal gland?

  Information about day length and light
• What happens to melatonin production during the night?

  Pineal gland increases melatonin production
• What did DeCoursey et al (2000) study?

  SCN connections in chipmunks
• What was the outcome of DeCoursey et al's study?

  Sleep/wake cycle disappeared in chipmunks
• What happened to the chipmunks after their SCN connections were destroyed?

  Many were killed by predators
• What did Ralph et al (1990) breed?

  Mutant hamsters with a 20-hour cycle
• What happened when SCN cells were transplanted into normal hamsters?

  Their cycles defaulted to 20 hours
• What does research suggest about circadian rhythms beyond the SCN?

  • Numerous circadian rhythms exist in organs and cells
  • Damiola et al (2000) showed feeding patterns alter liver rhythms
  • Suggests complex influences on sleep/wake cycle
• What is the interactionist system in biological rhythms?

  • Total isolation studies are rare
  • Pacemakers and zeitgebers interact in daily life
  • Isolating influences lowers research validity
• What are exogenous zeitgebers?

  External factors affecting biological rhythms
• How does light affect the SCN?

  It can reset the SCN
• What indirect influence does light have on the body?

  Controls hormone secretion and blood circulation
• What is the initial sleep schedule of newborn babies?

  Random sleep schedule
• When do babies begin to develop circadian rhythms?

  At about 6 weeks
• How are babies' rhythms influenced by parents?

  Entrained by parents' schedules at 16 weeks
• What do environmental observations reveal about sleep patterns?

  • People in extreme light conditions have varied patterns
  • Arctic circle residents maintain similar patterns year-round
  • Suggests endogenous pacemakers override light changes
• What case study did Miles et al (1977) recount?

  A blind man with a 24.9-hour rhythm
• What does the case study suggest about social cues?

  They are not effective in resetting biological rhythms
• What was the effect of social cues on the blind man's rhythm?

  His cycle could not be adjusted
• What is a limitation for Endogenous Pacemakers?

  • Beyond the Master Clock
  • Research has revealed that there are numerous circadian rhythms in many organs and cells in the body
  • Damiola et al (2000) demonstrated how changing feeding patterns in mice could alter the circadian rhythms of cells in the liver by up to 12 hours, whilst leaving the SCN unaffected
  • This suggests other complex influences on the sleep/wake cycle
• What is a limitation for Endogenous Pacemakers?

  • Interactionist System
  • Total isolation studies are extremely rare
  • 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 pacemnakers, the lower the validity of the research
• What is a limitation for Exogenous Zeitgebers?

  • Environmental Observations
  • The experience of people who live in different places where there is very little darkness in summer and very little light in winter tell a different story than the usual narrative
  • People who live within the Arctic Circle have similar sleep patterns all-year around, despite spending almost 6 months in almost total darkness
  • This suggests that the sleep/wake cycle is primarily controlled by endogenous pacemakers that can override environmental changes in light
• What is a limitation for Exogenous Zeitgebers?

  • Case Study Evidence
  • 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