Discuss endogenous pacemakers and exogenous zeitgebers

Biological rhythms are attuned to cyclic changes by an internal biological clock, known as an endogenous pacemaker. One type of biological rhythm that is controlled by the endogenous pacemaker the suprachiamatic nucleus (SCN) is our circadian sleep-wake cycle. 

The SCN lies in the hypothalamus of the brain and obtains information about light from the eye retina via the optic nerve. This happens even when our eyes are shut, because light penetrates the eyelids. Neural pathways allow the SCN to send a signal to the pineal gland to increase production of melatonin at night, which helps to induce sleepiness through inhibition of brain mechanisms that promote wakefulness. Supporting that SCN is involved in the process of the sleep-wake cycle, Stephan and Zucker (1972) found that in the destruction of the SCN eliminates the circadian pattern of sleep-wake and locomotion in rats. 

Furthermore, the evolutionary approach claims that the biological rhythms have adaptive value in order to aid species survival. Supporting this idea, Decoursey et al (2000) removed the SCN in 30 chipmunks and had a control group where no lesions were made. After 80 days, the SCN removed chipmunks had been killed by weasels, and the reason for this could be because the chipmunks remained wake in their burrows and the weasels could hear the noise and were able to locate the chipmunks. 

There are two issues which challenge the above research into endogenous pacemakers; one is that harm to animals has been involved in the research which questions the justification of the experiment due to animal rights issues. And secondly, as the research was conducted on animals there is a problem of generalizability to humans. Human’s biological systems differ from that of chipmunks and rats, meaning the findings lack in external validity therefore not useful to apply to human SCN research.   

Biological rhythms also need to be fine-tuned by external cues, also known as exogenous zeitgebers in order to remain in time with the external world. The dominant exogenous zeitgeber in humans is light. Light can reset the pacemaker SCN and can also reset other internal clocks such as CRY (cryptochrome), which is part of a protein clock. Recent research has found that even artificial lighting can influence biological rhythms such as by Campbell and Murphy (1998)found that if you shine light on the back of participants knees this shifted their circadian rhythms. 

The fact that we live in an artificial world may mean that it could have negative consequences. For example, Stevens (2006) suggested that exposure to artificial lighting disrupts circadian rhythms and thus disrupts melatonin production, which might explain why women in industrialised societies (usually well-lit environments) are more likely to develop breast cancer. 

However, the suggestion of correlation between artificial light and breast cancer does not mean causation as the development of cancers can be caused by a number of factors not just artificial light such as diet and genetics. 

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(expand: idea that melatonin can help to cure disruption of biological rythms, help reset of endogenous pacemaker)

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expand: Idea that methods of treatment such as melatonin injections can be used to help aid people suffering from primary insomnia

expand: state how its good for treatment of illnesses by its reductionist approach, however in some cases it could be due to social influences not biological causing the onset of insomina, so e.g depression, secondary insomina