circadian rhythm (sleep/wake cycle)

• circadian rhythms 


  follow a 24-hour cycle: e.g. the sleep-waking cycle

  • there must be an endogenous pace maker/ internal body clock that keeps this pattern and we are aware of conflicts between the the EP and exogenous zeitgebers like light and social cues
• endogenous pacemakers 


  internal body clocks that keep biological processes to time
• exogenous Zeitgebers 


  external cues that entrain internal body clocks, meaning altering body clocks to match the environment
• suprachiasmatic nucleus (SCN)

  • endogenous pace maker for sleep-wake
  • closely linked to the pineal gland
  • part of hypothalamus
  • located at optic chiasm
• effect of endogenous pacemakers on sleep wake cycle 


  SCN: main endogenous pacemaker. It controls other biological rhythms, links to other areas of the brain responsible for sleep/arousal.

  it receives information about light levels (an exogenous zeitgeber) from the optic nerve - sets the circadian rhythm so that it is in synchronisation with the outside world, e.g. day and night.

  it sends signals to the pineal gland, increasing production of melatonin inducing sleep. The SCN and pineal glands work together; however, their activity is responsive to the external cue of light.
• Siffre (1975)


  thought circadian rhythm to be free running without entrainment

  suffer tested this

  • spent 6 months in cave with no natural light or other EZ's like clocks/cues
  • his body clock maintained 25 hour cycle, longer than expected 24hrs
  • suggesting EP for sleep wake cycle is free running but needs entrainment to stick to 24hrs
• criticism of natural light isolation study


  psychologists have criticised use of artificial lights in isolation experiments , suggesting artificial lights disrupted measurements of free running circling rhythm by acting as an EZ.

  • in study controlling artificial light Czeisler found the circadian rhythm was closer to 24hrs
• evidence SCN is EP (Ralph)


  golden hamsters with tau mutation have circadian rhythms of 20 hours compared with 24 hours for normal golden hamsters

  • when SCN from normal hamsters was transplanted into brain of Tau mutant hamsters, they gained a sleep wake cycle of 24 hours
  • suggests SCN is main endogenous pacemaker
• effect of exogenous zeitgebers on sleep-wake

  • The most important zeitgeber is light, responsible for resetting the body clock each day, keeping it on a 24hr cycle.
  • The SCN contains receptors that are sensitive to light and this external cue is used to synchronise the body’s internal organs and glands. Melanopsin, which is a protein in the eye, is sensitive to light and carries the signals to the SCN to set the 24-hour daily body cycle. In addition, social cues, such as mealtimes, can also act as zeitgebers and humans can compensate for the lack of natural light, by using social cues instead.
• (weakness) biologically reductionist

  • Despite all the research support for the role of endogenous pacemakers and exogenous zeitgebers, the argument could still be considered biologically reductionist.
  • it only considers a singular biological mechanism and fails to consider the other widely divergent viewpoints.
• (strength) practical applications 


  psychological understanding of negative impacts of shift work and will hopefully lead to solution.

  This suggests that there may be practical uses in an improved understanding of the effects of desynchronisation. This in turn has economical implications, in terms of companies who employ shift-workers making the effort to revise their policies in order to reduce days taken off sick.
• (weakness) lacks validity


  Siffre's results may not be generaliseable to the population, as individual differences in the duration and stages of circadian rhythms has been shown, results may lack ecological validity.

  Siffre himself noted that as he grew older, his endogenous pacemaker ticked at a slower rate, which may have acted as an uncontrolled confounding variable in his investigations.

  This limits the extent to which the findings represent the experiences of the general population.
• (weakness) individual differences Duffy et al. (2001

  • found that ‘morning people’ prefer to rise and go to bed early (about 6am and 10pm) whereas ‘evening people’ prefer to wake and go to bed later (about 10 am and 1 am). This demonstrates that there may be innate individual differences in circadian rhythms, which suggests that researchers should focus on these differences during investigations.