Essay
Biopsychology essay - the role of endogenous pacemakers in the control of circadian rhythms.
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High band 4 answer for a 16 mark essay for the biorhythm topic (specifically circadian rhythms) of biopsychology.
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Discuss the role of endogenous pacemakers in the control of circadianrhythms
A circadian rhythm is a biological rhythm with a cycle length of about 24 hours, one
example of a circadian rhythm is the sleep/wake cycle. Circadian rhythms are controlled
by endogenous pacemakers. These are the biological ‘clocks’ in the brain controlling
biological rhythms. In mammals this is the suprachiasmatic nucleus (SCN) and the
hypothalamus. In the sleep/wake cycle, a lack of light stimulates the suprachiasmatic
nucleus which then sends a signal to the pineal gland to release melatonin, causing brain
activity to decrease resulting in sleep. When light is present the opposite occurs, light
stimulates the suprachiasmatic nucleus causing it to send a signal to the pineal gland
that inhibits the release of melatonin and signals for the release of cortisol.
Research into the role of endogenous pacemakers involved investigating the effect on
biological rhythms when a person is free from any external cues, such as time, light, etc.
This was investigated using temporal isolation studies such as Michel Siffre’s temporal
isolation study (1972) where he spent 7 months living inside a cave. During the study
Siffre’s sleep/wale cycle lengthened to ~24.9 hours (slightly longer than a typical day).
This shows that circadian rhythms persist despite the isolation from natural light,
demonstrating the existence of endogenous ‘clocks’ (endogenous pacemakers).
However, this study also shows the importance of external cues (exogenous zeitgebers)
on the sleep/wake cycle as the ‘clock’ was not entirely accurate (Siffre’s sleep/wake cycle
had lengthened to ~24.9 hours long) though still following a regular schedule. The
presence of exogenous zeitgebers entrains the endogenous pacemakers. A weakness of
the study is the use of 1 participant, making it hard to generalise. However later research
(Aschoff and Wever 1976) completed with more participants gave similar results, adding
generalisability and validity to the research. Aschoff and Wever (1976) convinced a group
of participants to spend 4 weeks in a WW2 bunker deprived of any natural light. The
results showed that all but 1 of the participants (whose sleep/wake cycle had extended to
29 hours) showed a circadian rhythm of 24-25 hours. Both Siffre’s and Aschoff and
Wever’s research suggests that the natural length of the sleep/wake cycle is slightly
longer than 24 hours but is entrained by exogenous zeitgebers associated with the 24-
hour day, e.g., number of daylight hours, typical mealtimes, etc.
Another temporal isolation study is Folkard et al. (1985). During the study a group of 12
people agreed to live in a dark cave for 3 weeks, going to sleep when the clock said
11.45pm and waking up when it said 7.45am. Over the course of the study the
researchers sped up the clock without the participants knowledge. A 24-hour day
eventually only lasted 22 hours. The results of the study showed that none of the
participants were able to adjust comfortably to the new schedule. This suggests the
existence of a strong free running circadian rhythm that is not easily overridden by
external changes. It also shows the strength of the circadian rhythm as a free running
cycle and questions the extent to which it can be overridden by exogenous zeitgebers.
However there may be methodological issues with all 3 studies as in these studies
participants were only isolated from natural light but not dim or artificial light. This
assumes that dim or artificial lighting does not affect circadian rhythms the same way
natural light would.
Another way research into the roles of exogenous pacemakers was conducted was
through the use of animal studies, such as Morgan (1995) mutant hamster’s study.
During the study hamsters were bred to produce ‘mutant hamsters’ (ones that had
circadian rhythms of 20 hours instead of 24 hours). The suprachiasmatic nucleus of these
hamsters was then transplanted into the brains of normal hamsters, which resulted in the
hamsters displaying the mutant rhythms. This supports the role of suprachiasmatic
nucleus in mammals as it suggests that the suprachiasmatic nucleus is the basis of the
endogenous pacemaker and the circadian rhythms. However, it is important to recognise
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