PSYA3 - Endogenous pacemakers and Exogenous zeitgebers

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DISCUSS THE ROLE OF ENDOGENOUS PACEMAKERS AND EXOGENOUS ZEITGEBERS IN THE SLEEP-WAKING CYCLE AND AT LEAST ONE OTHER BIOLOGICAL RHYTHM (25 marks) The sleep-wake cycle is an example of a circadian rhythm. These are rhythms that last for 24 hours. The circadian rhythm may be controlled by endogenous pacemakers (internal innate mechanisms such as the suprachiasmatic nucleus) or/and exogenous zeitgebers (external factors such as the light/dark cycle) A case study of a man who was blind from birth found that he had a 25 hour sleep/wake cycle despite exposure to clocks and radios that acted at time cues.


This shows we use the exogenous zeitgeber light to synchronise our sleep-wake cycle to 24 hours, the natural cycle of day and night. A natural study into residents in the Arctic Circle found that in the summer months when the sun didn't set they still sleep for 7 hours a day. This suggests that there are other factors than light that act as zeitgebers, such as social life and clocks. An Infradian rhythm is a biological bodily rhythm that is longer than a day but shorter than a year. The most obvious example is the female menstrual cycle which may be controlled by hormones or pheromones.


This is very similar to the way sleep-wake cycle is affected by light. Case studies such as the blind man and the cave study are not very scientific as they only have one participant. It is hard to control all the variables and you cannot generalise the results. But studies on the sleep-wake cycle and the menstrual cycle all show that the endogenous pacemaker is affected by external zeitgebers. This is an example of nature-nurture interaction, as the innate pacemaker is affected by the environment. This study of endogenous pacemakers is an example of biological reductionism as it explains sleep-waking using biological pacemakers in the brain. As research shows that these can be affected by external zeitgebers, you cannot explain biological rhythms just using biological pacemakers.

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