Circadian Rhythms

• Circadian Rhythms - what?


  Circa = about 

  Dia = a day 

  Biorhythm with a daily frequency - the circadian rhythm lasts for about 24 hours

  e.g. sleep/wake cycle
• 2 examples of circadian rhythms?


  • There are several important types of circadian rhythms such as the sleep/wake cycle.
  • The sleep/wake cycle is an example of a circadian rhythm. Drowsiness/alertness is the internal change whereas darkness/daylight is the zeitgeber. 
  • Body temperature is another example of a circadian rhythm.
  • Our core body temperature is as high as 38 degrees celsius at 6-7pm and as low as 36 degrees celsius at 4-5am.
• What are circadian rhythms made & kept by


  Although circadian rhythms are endogenous ("built-in", self-sustained), they are adjusted (entrained) to the local environment by external cues called zeitgebers, commonly the most important of which is daylight. 
• What is the sleep wake cycle governed by?

  Sleep/wake cycle governed by internal and external mechanism:

  • Exogenous zeitgebers - the fact we feel drowsy when it's night-time and alert during the day shows the effect of daylight.
  • Endogenous pacemakers - a biological clock ‘left to its own devices’ without the influence of external stimuli (e.g. light) is called ‘free running’.
  • The basic rhythm is governed by the SCN. The SCN lies just above the optic chiasm which provides information from the eyes about light.
  • Exogenous zeitgebers (light) can reset SCN.
• Sleep/Wake Cycle diagram?

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  A) ?

  
• Psychologists involved in Sleep/wake cycle?

  •  Michael Siffre
  • Aschoff & Wever (1976)
  • Folkard et al (1985)
• Outline the PROCEDURE case study of Michael Siffre:


  Procedure?

  • French caver Michael Siffre has spent several long extended periods underground in dark caves to study the effects on his own biorhythms (examine the effect of free-running biological rhythms). He spent 2 months (in 1962) and 6 months (in the 1970s).
  • He was deprived of exposure to natural light and sounds but had access to food and drink
• Outline the FINDINGS case study of Michael Siffre?


  • After spending 2 months in a cave in the southern Alps he emerged believing it to be mid-August when it was mid September. This is because his free-running biorhythm settled down to one that was around 24.5-25 hours.
  • In each case study, Siffre’s free-running circadian rhythm settled down to about 25 hours. Importantly he did have a regular sleep/wake cycle. Siffe demonstrated a free-running circadian rhythm of about 25 hours
• Aschoff & Wever (1976) - PROCEDURE & FINDINGS


  Procedure:

  • Studied ppts spent 4 weeks in a WW2 bunker where they deprived of natural light

  Findings:

  • All (but one) emerged with circadian rhythm lasting between 24 & 25 hrs
  • But one had circadian rhythm extended to 29 hours - anomaly
• Aschoff & Wever (1976) - CONCLUSION


  Conclusion:

  • Suggesting (along with Siffre) our natural sleep/wake cycle is slightly longer than 24 hours but it is entrained by exogenous zeitgebers associated with our 24 hour day (like number of daylight hours, typical meal times etc).
  • BUT, this research also shows zeitgebers like light are key because the internal clock not perfectly accurate: varying from day to day.
• Folkard et al (1985) - PROCEDURE & FINDINGS


  PROCEDURE:

  • Studied a group of 12 pps who agreed to live in a dark cave for 3 weeks
  • They retired to bed when the clock said 11.45pm and rose when it said 7.45am 
  • Unbeknown to the pps the researchers gradually sped up the clock so that a 24 hour day only lasted 22 hours over the course of the study 

  FINDINGS:

  • Only 1 pp was able to adjust comfortably to the new regime
• Folkard et al (1985) - CONCLUSION


  CONLUSION:

  • Shows circadian rhythms can only be guided to limited extent by exogenous zeitgebers.
  • Suggesting the existence of a strong free-running circadian rhythm that by exogenous zeitgebers CAN'T easily be overridden by changes in external environment (not controlled by exogenous zeitgebers)
  • Showing endogenous pacemakers stronger than exogenous zeitgebers:
  • However, there are exceptions as individual differences need to be taken into account
  • Zeitgebers have little influence - circadian rhythm only guided to a certain extent by zeitgebers.