Outline and evaluate research on circadian rhythms with reference to endogenous pacemakers and exogenous zeitgebers. (8+16 marks) Circadian rhythms are biological cycles that last around 24 hours, such as the sleep/wake cycle which is usually facilitated by time checks and regular events like meal times. There is a free running cycle controlled by an endogenous pacemaker working as a body clock which is internal to our biological system, such as the suprachiasmatic nucleus which receives information about light and dark directly from the retina of the eye and the pineal gland, which releases melatonin in relation to fading light. Melatonin acts upon the raphe nuclei which in turns produces serotonin, a chemical with an inhibitory or slowing down effect on the central nervous system. Also, circadian rhythms can be controlled by exogenous zeitgebers i.e. external time givers, such as light, temperature and social cues. Another circadian rhythm is body temperature, which is at its lowest at around 4.30 am around 36C and peaks at around 6,00am at around 38C. The core body temperature also dips after lunch, however this is not due to the fact that we have just eaten, it just naturally occurs in the body. This dip in body temperature is related to many countries having an afternoon siesta. Evidence to support the role of endogenous pacemaker of light with regards to circadian rhythms comes from Siffre (1962) who carried out research using himself as the sole participant. He spent two months in an underground cave, without time th

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cues and emerged on the 27 of September, but he thought it was the 20 of August. This suggests that the absence of exogenous zeitgebers, affected his perception of time, therefore giving evidence that endogenous pacemakers control our circadian rhythms. He then carried out further research to see if the absence of exogenous zeitgebers would affect his sleep- wake cycle. He spent six months in a cave, with no time cues, but artificial lights. He settled into a sleep wake cycle of around 25 hours. This therefore gives reliability and validity to the view that circadian rhythms are set by endogenous pacemakers, and are not dependent on the exogenous cue of light. This research allows us to carry out causal relationship experiments into the importance of endogenous pacemakers.

Unfortunately, Siffre has been the only researcher in his experiments and therefore the evidence to suggest that endogenous pacemakers control our circadian rhythm of the sleep wake cycle, cannot be generalised to the population. Therefore does not take into account individual differences such as culture, age and gender when investigating the sleep wake cycle. However, Siffre's research methods have been criticised by Cszeiler (1999) who suggested that previous studies Siffre argued that artificial light would not affect the circadian rhythm. He carried research on 24 men and women who lived for a month in very subdued light, and during which tried to place them in an artificial 28hr sleep-wake cycle. , during which their body temperature and chemistry was monitored. They found that the human sleep clock operates on a 24hr 11 minute schedule not 25 hours. Suggesting that even artificial light and exogenous zeitgebers can have an impact on circadian rhythms. To also suggest that we have a 25 hr. sleep- wake cycle would be an evolutionary disadvantage, as non-human animals consistently show a 24 hr. cycle. It would obviously be an evolutionary advantage for survival and adaptation to have a 24 hr. sleep- wake cycle. Furthermore, the view that circadian rhythms such as the sleep wake cycle are controlled only by endogenous pacemakers such as the SCN and the pineal gland is reductionist, and human behaviour is more complex than innate biological drives. Individuals do have free will over their sleep wake cycle, and can control this with the use of exogenous zeitgebers. However, sometimes our biological rhythms cannot be controlled, expressing the view that our circadian rhythm is biologically determined. This can be further validated by a study of a blind man by Miles et al (1977).The man was blind from birth and had a circadian rhythm of 24.9 hours. He was exposed to various exogenous zeitgebers such as clock and social cues, yet found great difficulty in reducing his internal pace. This made it very difficult for him to function and as a result he had to take stimulants in the mornings and sedatives at night in order to get his biological rhythm in time with the rest of the world. In conclusion with regards to circadian rhythms and the role of endogenous pacemakers and exogenous zeitgebers maybe we should look at the interactionist view of nature and nurture influencing each other with regards to circadian rhythms such as the sleep wake cycle and body temperature.

Your challenge. 1) Identify the A01 and A02 with two different coloured highlighters in the margin. 2) Highlight the key terms of A01 in the same colour. 3) Highlight the studies and IDAS names and terms in the A02 Colour. 4) Condense this essay into a 24 bullet point plan. 8 A01 points/ 16 A02 points.

A01 1 2 3 4 5 6 7 8

A02 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16