Endogenous Pacemakers and Exogenous Zeitgebers

• Endogenous pacemakers


  Internal body clocks that regulate many of our biological rhythms, such as the influence of the suprachiasmatic nucleus (SCN) on the sleep/wake cycle.
• Sleep/wake cycle


  A daily, naturally occurring 24-hour circadian rhythm of sleep and wake states regulated by a biological clock
• The suprachiasmatic nucleus


  A cluster of neurons in the hypothalamus in the brain that governs the timing of circadian rhythms.
• Hypothalamus


  A neural structure lying below the thalamus; it directs several maintenance activities (eating, drinking, body temperature), helps govern the endocrine system via the pituitary gland, and is linked to emotion and reward.
• DeCoursey et al (2000)


  - Destroyed SCN connectors in brains of 30 chipmunks.

  - Returned to habitat and observed for 80 days

  - Sleep/wake cycle disappeared and significant amount were killed by predators (as were awake and vulnerable at wrong time).

  - Shows influence of SCN.

  
• Ralph et al (1990)


  - Bred 'mutant' hamsters w/ 20hr sleep/wake cycle

  - When SCN cells from mutant hamsters transplanted into brains of normal hamsters - cycle of second group changed to 20hrs.

  - Shows influence of SCN

  
• Limitations of endogenous pacemakers and the sleep/wake cycle

  Beyond the master clock

  - May obscure other body clocks.

  - Research has revealed that there are numerous circadian rhythms in many organs and cells in the body.

  - Suggests other complex influences on the sleep/wake cycle.

  
  

  Interactionist system

  - Total isolation studies (siffre) are extremely rare.

  - Suggests the more researchers attempt to isolate the influence of internal pacemakers, the lower the validity of the research.
• Exogenous zeitgebers


  - Cues from the environment that play an important role in regulating time and the circadian rhythm.

  - They are the info for the pacemakers and regulate internal body clock (light, season)
• Campbell and Murphy


  Light detected on skin even when not by eyes, 15 ppts woken at various times and a light pad was shone on back of knees, researcher produced deviation in sleep/wake cycle of up to 3 hours
• Limitations of exogenous zeitgebers and the sleep/wake cycle

  Environmental observations

  - Exogenous zeitgebers don't have the same effect in all environments.

  - The experience of people who live in places where there is very little darkness in the summer and very little light in the winter tell a different story from the usual narrative.

  - Suggests the sleep/wake cycle is primarily controlled by endogenous pacemakers that can override environmental changes in light.

  
  

  Case study evidence

  - 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.

  - Suggests that social cues alone aren't effective in resetting the biological rhythm.