Evaluation

• limitation: endogenous pacemakers and exogenous Zeitgebers interact

  only in exceptional circumstances do endogenous pacemakers free-run unaffected by exogenous zeitgebers. Total isolation experiences (e.g. Siffre's study) are extremely rare and present an unrealistic view of how the system works. endogenous pacemakers and exogenous zeitgebers interact in real life- it makes so sense to separate them just for research purposes
• limitation: use of animals

  issue in generalising findings from research into the sleep/wake cycle from animal studies because cognitive factors may be more significant in humans. a ore disturbing issue (e.g. in the decoursey et al. study) is the ethics of such research- animals were exposed to great harm and potential risk when returned to their natural habitat. whether what we learn from investigations on biological rhythms justifies the aversive procedures involved is a matter of debate
• limitation: methodological issues in exogenous zeitgeber research

  Campbell and Murphy's study has yet to be replicated and is criticised because there may have been some light exposure to participants' eyes- a major confounding variable. also, isolating one exogenous zeitgeber (light) in this way does not give insight into the many other zetigebers that influence the sleep/wake cycle. this suggests that some studies may have ignored or underplayed the way in which different exogenous zeitgebers interact
• limitation of SCN research: may obscure other body clocks

  body clocks (peripheral oscillators) are found in many organs and cells (e.g. lungs, liver, skin, pancreas). They are highly influence by the actions of the SCN but can act independently. Damiola et al. showed how changing feeding patterns in mice altered circadian rhythms of cells in the liver for up to 12 hours, leaving the rhythm of the SCN unaffected. This suggests there may be many other complex influence on the sleep/wake cycle, aside from the master clock (SCN)