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Describe and Evaluate Circadian Rhythms

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    Describe circadian rhythms (8marks) Circadian rhythms occur every 24 hours; an example of a circadian rhythm is the sleep-waking cycle. We are diurnal animals who are active during the daytime and asleep at night, other animals are nocturnal they are active at night but asleep during the day. The circadian rhythm depends on the interaction of physiological and psychological processes to be tuned into the sleep-waking cycle so energy is provided when needed.

    As diurnal humans we have a fairly stable sleep pattern with the time we go to sleep and the time we wake up, this consistency suggests an internal mechanism controls sleep, endogenous pacemaker. However, this can be overridden by external factors, exogenous zeitgebers. The endogenous pacemaker that works as an internal body clock to regulate biological rhythms of the sleep-waking cycle is the suprachiasmatic nucleus (SCN, a small group of cells in the hypothalamus).

    The SCN has a direct pathway to the retina of the eye, in which light enters. Light is an exogenous zeitgeber which also has an influence on the rhythm due to being synchronised with the SCN. The SCN is also connected to the pineal gland by a neural pathway; the pineal gland is where the hormone melatonin is manufactured. Melatonin affects sleep patterns, the more melatonin released due to low levels of light the more sleep, when melatonin levels drop due to high levels of light the individual wakes.

    However, sleep/wake cycles can be maintained in the absence of light as the SCN is free-running, it follows its own innate rhythm and continues to follow a pattern of circadian activity even when isolated from the rest of the brain. Discuss research into circadian rhythms (16marks) One of the earliest scientific studies on endogenous pacemakers was conducted by Stephan and Zucker (1972). They wanted to investigate the effects of damage to the SCN (of rats) on circadian rhythms; they deliberately damaged the SCN via surgical controlled lesions.

    In the lab, the rats that are nocturnal animals were exposed to 12 hours of light and 12 hours of darkness. Those whose SCN was damaged showed abnormal patterns, suggesting the SCN is a key pacemaker. Stephan and Zucker (1972) successfully carried out a controlled and scientific study of endogenous pacemakers that had many of the features that Popper identified as essential to the scientific process. The use of the laboratory experiment provided control which enables them to test the effect of the damaged SCN on circadian rhythm.

    This control provided high internal validity which gives credibility to the results drawn. The highly controlled experiment means that the study is easily replicable. The use of rats provided a stepping stone towards developing a hypothesis for humans. The next step for this study is to replicate the study however using a mammal more similar to humans, such as monkeys. Popper believed that observations should be collected from which a hypothesis can be formulated and tested under controlled conditions.

    The reliability of the rats study provides this scientific process starting point to further conduct research that will apply to the human population. Fellow psychologists also studying circadian rhythms also found supporting results, Zucker, Boshes and Dark 1985 for example found that damage to SCN of squirrels also disturbed the animal’s circadian rhythms. This further evidence provides intra rater reliability to Stephan and Zucker’s study as both sets of research found the same results.

    The use of rats was effective in providing the study with objectivity, there would have been no experimenter effects and no demand characteristics. However, there are ethical issues existing due to the large number of 14 out of 25 rats that died when the SCN was damaged. The use of non-human animals in research involves a balance between the values of the findings against the stress to the animal. Bateson (1986) suggested three dimensions when evaluating non-human research: the degree of suffering involved, the scientific quality of the research proposal and the potential benefit to society of the results.

    When using non-humans there needs to be justification to inflict this kind of sacrifice, due to the number of rats that did die during this study is a major concern and as Stephan and Zucker’s study was conducted in 1972 before the regulations were tightened, it is unlikely their study would be permitted today. To explain the control of biological rhythms by just referring to endogenous pacemakers such as the SCN is an example of biological reductionism. This is a limited approach as it is partial and does not take account of the interaction of endogenous pacemakers with exogenous zeitgeber.

    The psychologists assume that the damage to the SCN is the only causation of the disturbed sleep there is no consideration that the sleep-waking cycle could be a more complex mechanism. The SCN is connected to the pineal gland which could be the underlying cause of the disturbed sleep, due to the damage of the SCN this connection may mean that melatonin is no longer secreted efficiently. This would suggest that the SCN is only a factor within a larger pathway. Michel Siffre however did take into account the interaction of endogenous pacemakers and exogenous zeitgebers.

    Siffer, a French geologist, spent six months in a cave with no natural light; his biological rhythms became free-running in the absence of light. The cave was artificially lit and Siffre had a phone to the outside world to turn light on or off. He had no watch and ate and slept as he felt. His physiological measurements were recorded; body temperature, heart rate, blood pressure and his sleeping and waking patterns. He found that waking circadian rhythms extended from the normal 24 hours to between 25 and 32 hours. When he emerged on the 179th day he thought it was only the 151st day.

    His body temperature circadian rhythm was more stable; it extended slightly to 25hours but remained consistent. This means that his body temperature circadian rhythm was desynchronised with his sleep-waking circadian rhythm. Under normal conditions our sleep-waking cycles and our body temperature cycles are synchronised, we go to sleep when temperature is falling and wake up when temperature is rising. From this case study we can conclude that endogenous pacemakers and exogenous zeitgebers need to interact to be synchronised.

    We can confirm that in the absence of light the SCN becomes free-running and it is desynchronised with exogenous zeitgebers. Due to the temperature remaining consistent it is implied that different pacemakers in the brain for the sleep-waking cycle SCN and temperature are in a different place. Overall it can be concluded that humans are very much biologically determined, even if the external environment is dramatically different from the normal environment the body will run as it did before or near to.

    The use of a case study means that the results cannot be generalised to anybody else other than Siffre as he could behave in a very different way to any other individual. Although the study provides great detail, Siffre being a geologist could initiate great bias; Siffre being in a cave would be an everyday norm to him however another individual could react extremely differently. This lack of generalisation causes the study to have low external validity as none of the conclusions drawn from the research could be applied to any other person. Siffre being very aware of the nature of the tudy makes it non objective, knowing that his sleep-waking pattern is being measured he could have tried to sleep and wake when he felt was the correct time, due to his knowledge this introduced demand characteristics into the study, which could mean that the real effect of no light as the exogenous zeitgeber could actually have a different effect on the circadian rhythm rather than the results found for Siffre. During the study at 80days in, Siffre reported feelings of depression, suicidal thoughts and memory problems, post study he also reported long-lasting psychological problems.

    A symptom of depression is either insomnia or hypersomnia and as Siffre was suffering depression what could have actually been measured are the effects of depressing on the circadian rhythm rather than the lack of light. This uncertainty to what actually is being measured causes a lack of internal validity, it is not definite that the independent variable is light, it could be that other extraneous variables such as noise as an example also interfered with the sleep-waking cycle. Not only does the depression question the study’s findings but the psychological problems associated with the study raise extreme ethical issues.

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