AO3

• One limitation of research into endogenous pacemakers is that they cannot be studied in isolation 


  Total isolation studies such as Siffre’s cave study are extremely rare. Siffre’s study may have lacked internal validity due to use of artificial light which could have reset his biological clock. In everyday life, pacemakers and zeitgebers interact and it may make little sense to separate the two for the purpose of research .
• One weakness of research on the SCN is that it may overlook other body clocks. Studies have found that many organs and cells like those in the lungs, liver and skin, have their own circadian rhythms. These ‘peripheral clock‘ can also work on their own. Animal study on mice found that changes in feeding patterns could alter circadian rhythms of cells in the liver whilst leaving the rhythm of the SCN unaffected
• One limitation of exogenous zeitgebers is that they do not have the same effect in all environments. For instance, people who live in the Artic Circle have similar sleep patterns all year round despite spending around 6 months in almost total darkness. Suggests sleep/wake cycles is primarily controlled by endogenous pacemakers that can override environmental changes in light
• Case study evidence that 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 despite exposure to social cues