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  • Created on: 31-12-12 14:33

A01 Siffre cave

Circadian rhythms are biological cycles which last 24 hours, one of which is the human sleep-wake cycle. Research suggests that biological rhythms involve internal body clocks (or endogenous pacemakers) and environmental stimuli (or exogenous zeitgebers) such as light.

One such study, Siffre (1979), spent 179 days in a cave in Texas. There was no natural light, and in these conditions the biological rhythm of the sleep-wake cycle become free running, meaning it had to rely on just the endogenous pacemaker. His sleep-wake pattern was continually recorded and he was allows to turn artificial light on and off. The findings showed that Siffre’s sleep-wake cycle increased from the normal 24 hours to between 25 and 32 hours. He concluded that in the absence of exogenous zeitgeber, light, the endogenous pacemaker can regulate circadian rhythms but not accurately. 

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A02 Controlled variables, clear relationship

This is good evidence for the existence of body clocks, as Siffre controlled key variables to observe the effects on sleep-wake cycle which demonstrated a clear causal relationship between light and the body clock. The evidence also suggests that body clocks need light onset to trigger them at the right time. 

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A02 Case study, morning and evening people

However, as this was a single case study of one man, the findings cannot be generalized to the rest of the population as there may be significant individual differences in how we react to the free running conditions. For an example Duffy et al (200) found that morning people prefer to rise early and go to bed early (6:00am to 10:00pm), whereas evening people prefer to wake and go to bed later (10:00am and 1:00am). Also there was only one participant, making the study also lacking in reliability. 

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A02 Achoffe and Weber, similar result

On the other hand, later research has supported Siffre’s study. Achoff and Weber (1965) found that the sleep-wake cycles of students kept in similar conditions to Siffre in an underground bunker also extended, to between 25 and 27 hours.

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A02 Artificial light, Czeisler et al, validity

Siffre was allowed to use artificial light as it was thought dim light, in contrast to daylight, would not affect the circadian rhythm. Recent research shows that this claim may be untrue, Czeisler et al (1999) altered participant’s circadian rhythms down to 22 hours and up to 28 hours just using dim lighting. Which suggests that Siffre’s findings may also lack in validity as the extraneous variable of artificial light may have caused the results to be inaccurate.   

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A01 Biological SCN

The sleep-waking cycle can also be explained by a biological approach. Studies have found that the suprachiasmatic nucleus (SCN) in the hypothalamus of the brain is linked to the circadian pattern of sleep-waking and locomotion, due to the destruction of SCN leads to the elimination of sleep-waking and locomotion in rats (Stephen and Zucker, 1972), showing its central role as a body clock.

Neural pathways connecting the SCN with the pineal gland allow it to control the release of the hormone melatonin from the pineal gland. 

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A02 Treatment

Research shows that the hormone, melatonin, is linked to the sleep-waking cycle as one of the possible triggers for sleep for an example brain melatonin levels increase in darkness and decrease with the onset of light. Some studies have also found that melatonin can be an effective treatment for circadian rhythms disrupted through jet lag. 

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A01 Desynchronised

Our body clocks can become desynchronised from the main zeitgeber, light. This leads to the disruption of our normal pattern of biological rhythms. Research into disruption aids understanding of the role of exogenous zeitgebers. Such disruption can be caused by jet lag and shift work, as people are trying to work into the night while their biological clock is trying to impose sleep or the biological clock is ahead or behind in the local time. 

A study which shows the effect of disruption on circadian rhythms was Czeisler et al (1982). They changed a company’s workers from a backwards shift rotation to a forwards rotation. Previously workers reported high levels of stress and health problems (research has shown disruption to biological clock can reduce the immune system).  After nine months workers reported less stress and fewer health problems, and productivity increased. 

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A02 Reductionist

Much research into circadian rhythms is related to the idea of the biological approach which concerns behaviour in terms of the structure of the brain, interactions between neurons and the effect of hormonal activity. However human behaviour is often more complex than this, which suggests the approach is very reductionist and deterministic. People can overdrive biologically determined behaviours by making choices about what they do.

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A02 Deterministic


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A02 Miles et al blind man

 On the other hand, sometimes this may not be possible; Miles et al (1977) study of a young man who was blind from birth had a circadian rhythm of 24.9 hours. He was exposed to various exogenous zeitgebers, yet found great difficulty in reducing his internal pace. In order to get his biological rhythm in time with the rest of the world, he had to result in taking stimulants in the mornings and sedatives at night. However as this is a case study, it lacks generalizability as this man had a rare circadian rhythm, but it does prove that not all people can overdrive biologically determined behaviour due to individual differences.   

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