Discuss the role of endogenous and exogenous factors in the control of one or more circadian rhythms (16 marks)
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Biopsychology
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This essay includes evaluating studies by researchers such as Siffre and Morgan and is worth 16 marks. It discusses the biology behind circadian rhythms (e.g. the process of the pineal gland secreting melatonin) and is useful for any psychology student.
Discuss endogenous and exogenous factors in the control of one or more circadian rhythm
(16 marks)
Our sleep wake cycle is a circadian rhythm as we sleep and wake in a regular 24 hour pattern. This is
controlled by both endogenous (internal) factors and exogenous (external) factors. Light levels are
exogenous factors, when natural light levels are low the optic nerve fires less frequently. The
suprachiasmatic nucleus (SCN) detects this and signals the pineal gland to release melatonin.
Melatonin is a hormone that decreases brain activity and makes us sleepy. When light levels
increase, so does activity on the optic nerve, the SCN detects this and signals the pineal gland to stio
releasing melatonin, the brain activity increases and we wake up. The SCN is thought of as being the
main endogenous pacemaker and is often called the body clock.
There is strong experimental evidence to support the claim that the SCN controls circadian rhythms.
Animal studies such as Morgan removed the SCN from hamsters and found that their normal
rhythms disappeared and they slept and woke randomly. Further he found that by transplanting the
SCNs from foetal hamsters into the lesioned ones, the rhythm was restored. This provides conclusive
proof that the SCN is the body clock. However, it may be wrong to generalise these findings to other
species like humans. Humans do have a SCN but also have a higher level of cognitive functioning so
might be more able to override biological instincts and be less affected by the loss of the SCN than
simpler mammals. It would also be unethical to test this out on humans in the same way as can with
animals.
Other research therefore investigates the effect of removing exogenous factors in order to establish
that humans do have an internal body clock. For example, Siffre lived in a cave for 7 months and his
patterns of activity were monitored. It was found that he deviated from the normal 24 hour rhythm
and settled on one between 25 and 26 hours (his day extended by just over an hour when he had no
exogenous factors). This shows that humans do have a biological body clock that can operate
independently of the environment, but under normal circumstances it is entrained by exogenous
factors like daylight levels. However, Siffre is a single case and it may be wrong to generalise from
one person to all humans because he may be unusual. However, subsequent similar studies show
similar results, although with differing rhythm lengths, each person lengthened their day when
removed from exogenous influences.
This research is very useful for people who work shifts. These are often people in important jobs and
who need to be alert at night when their body clock is causing the release of melatonin, decreasing
brain activity. Also, they need to sleep in daylight and will find it difficult to do their job well.
Understanding this process though allows us to advise them to use black out blinds during the day to
slow the optic nerve and get the SCB to signal to the pineal gland to release melatonin and to ensure
that there are high levels of light in the workplace at night to decrease melatonin.
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