PSYA3 - biological rhythms

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Outline and evaluate research on circadian rhythms

A biological rhythm is a cyclical variation over a period of time in physiological or psychological processes. Circadian rhythms last around 24 hours, for instance the sleep/wake cycle.

Research has been conducted to try and find out what causes the rhythms. Are they related to exogenous zeitgebers e.g. light and dark? Or are they inbuilt and would persist even if  external  stimuli are absent. These are endogenous pacemakers. Researchers are interested in what happens when exogenous cues are removed. Siffre was a french geologist who spent 2 months in a cave with no natural light, but had food, water and could communicate but had no way of finding out the time. His physiology and behavior stayed cyclical but becomes based on a day between 25 and 30 hours. Therefore the body must have an internal clock, (endogenous pacemaker).

However Siffre’s study is a case study as it is studying one person, himself and therefore it has unique features. Due to this his behaviour may not be typical of all people and therefore means it cannot be generalised. It is possible that there was an investigator bias as he was not only the investigator and the participant.

Aschoff and Wever conducted a study in support of Siffre’s. They placed participants in an underground bunker in the absence of any environmental and social cues. They found that most participants had circadian rhythms of between 24 and 25 hours although some were longer. This shows that the circadian rhythm is about 24 hours.

A study was also conducted by Folkard et al. who isolated participants from exogenous zeitgebers for 3 weeks, manipulating the clock they had so that only 22 hours passed a day. Out of the 12 participants, only one kept pace with the clock. Showing the strength of the circadian rhythm as a free running endogenous pacemaker.

However there are individual differences with circadian rhythms. One of such differences is the cycle onset. People seem innately different in terms of when they reach their peak. For example, Duffy et al. found that morning people prefer to rise early and go to bed early, whereas evening people prefer to wake up and go to bed late. Suggesting that there are individual differences.

There are also criticisms with the methodology of the research conducted into circadian rhythms. It was thought that dim light, in contrast to daylight, would not affect the circadian rhythm.

However recent research suggests that this is not likely to be the case. Czeisler et al. altered participants’ circadian rhythms down to 22 hours and up to 28 hours just by using dim light. Suggesting that dim light does affect the circadian rhythm.

Siffre went on to suggest that the body must have an endogenous pacemaker. Such as clock in the brain. There is a structure called the suprachiasmatic nuclei (SCN) in the hypothalamus to receive information about light and dark directly from the retina which synchronises our biological rhythms to a 24


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