Circadian rhythms

• What is a circadian rhythm?

  Last 24 hours
• What is an exogenous zeitgaber?

  External factor
• What is an endogenous pacemaker?

  Internal biological clock
• What is our bodies pacemaker?

  Suprachiasmatic nuclei
• Where do we find the SCN?

  Hypothalamus
• What is the term that means our internal clock runs independently?

  Free running
• Who did the cave study?

  Siffre (1962)
• What did Siffre do?

  • Spent 2 months living in a cave with no external cues to guide his rhythm (eg light)
  • His rhythm settled at 24.9 hour days
  • This is evidence for free-running endogenous circadian rhythms
• What are the strengths of Siffre’s study?

  • Controlled environment
  • Replicable therefore reliable
  • Evidence for free-running circadian rhythms
• What are some weaknesses of Siffre’s study?

  • Case study so may not be generalisable
  • Artificial light may have been used. Czeiser found this to be an exogenous zeitgaber, so the study lacks validity
• Why did Aschoff and Weaver (1976) find?


  Ps spent 4 weeks in a bunker deprived of natural light, and they all had an extended sleep wake cycle. This suggests our sleep wake cycle is constrained to 24 hours by exogenous zeitgabers
• Who did the bunker study?

  Folkard (1985)
• What did Folkard do?

  • Placed 12 people in a bunker for 3 weeks
  • They went to bed at 11:45pm and woke up at 7:45am
  • Throughout the study the researchers sped up the clock so their day only lasted 22 hours
  • Nobody could adjust to the new regime showing we must have an endogenous system
• How can night shift work be helped?

  If people are tired and having accidents during night shifts due to an increase in melatonin we can make brighter lights in hospitals etc
• What is chronotherapy?

  Treatment timing based on biological rhythms