Exogenous Zeitgebers and Endogenous Pacemakers

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Describe and evaluate research into circadian rhythms, with reference to endogenous pacemakers and exogenous zeitgebers (25 marks) Circadian rhythm: those that last around 24 hours. The two main examples of circadian rhythms include the sleep- wake cycle and the body temperature cycle. The circadian rhythm of the sleep wake cycle is controlled by endogenous pacemakers, which are essentially internal biology ‘clocks’ and also by external environmental cues, called exogenous zeitgebers e.g. light. (AO1) Research into circadian rhythms Siffre (1972): investigated the effect of the removal of the exogenous zeitgebers (light) on the circadian rhythm of the sleep wake cycle. He spent 7 months in an underground cave with no natural light. He continued to sleep and eat as normal, but didn’t know the time. It was found that his physiology and behaviour remained the same, but his circadian rhythm increased from 24 hours to 25 hours. It can be concluded that circadian rhythms persist despite the removal of exogenous zeitgebers. (AO1) Evaluation • Supported by research by Aschoff and Wever (1976) they placed participants in an underground WW2 bunker, in the absence of exogenous zeitgebers. They found the participants still had their 24/25-hour circadian rhythms, which provides evidence for the role of internal endogenous zeitgebers. (AO2) • Both studies use small sample sizes. In particular, Siffre’s study was a case study, which focused on one individual person. Therefore, the results can’t be generalised to the wider population, as it doesn’t account for individual differences. (AO3) • Another weakness is that Siffre’s study only accounts for biological influences on the circadian rhythm and so only represents the nature side of the nature-nurture debate. This provides an unrealistic view of human behaviour because in real life both nature and nurture
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