Discuss the role of endogenous and exogenous cues in the role of biological rhythms (24 marks)

An essay on endogenous cues and exogenous zeitgebers

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Chloe Blunden
Discuss the role of endogenous and exogenous cues in the role of circadian
rhythms (24 marks)
Endogenous pacemakers are our internal body clocks that help to manage our rhythms, such as
the suprachiasmatic nucleus, melatonin, the pineal gland and the hypothalamus which can be
influenced by exogenous zeitgebers (external factors) such as the sun rise, alarm clocks and bird
call. The most important endogenous pacemaker is the suprachiasmatic nucleus (SCN) is a bundle
of nerves located in the hypothalamus just above the optic chasm therefore it can gain input
directly from the eyes which allows it to be reset by the amount of light entering the eye. It is
shown to be important in Menaker et al's study on hamsters where they cut the hamsters SCN
and this caused a disruption to their circadian rhythms. A problem with this study is that we
cannot generalise the findings from hamsters to humans as we are biologically dissimilar. Also, this
animal study has caused harm to the animals as even when their NCS was returned via transplant,
their circadian rhythms still didn't return, which is unethical.
Some psychologists have claimed that the SCN is needed for survival and therefore has an
adaptive value when controlling rhythms. This statement is supported by DeCoursey's study that
took 30 chipmunks from their natural environments and removed their SCN, then released them
back into the wild. They found that after 80 days without their SCN, many of the chipmunks had
died when compared to a control group. This shows that endogenous pacemakers, especially the
SCN are important in our survival, although the study is weakened in the fact that it was done with
chipmunks, therefore we cannot generalise the findings to humans biological rhythms.
Another study that gives us an insight on endogenous pacemakers and exogenous zeitgebers was
Michel Siffre's study on the endogenous pacemakers in our body. He spent 6 months in a cave
that was fitted with complete natural lighting which therefore took away all the exogenous
zeitgebers that could influence his endogenous pacemakers. Inside the cave, Siffre recorded his
vital measurements, such as his brainwaves, temperature, heart rate and blood pressure etc. After
only a few days in the cave with no other physical contact with other humans and any other
exogenous zeitgebers, his internal body clock changed and he began having a 25 hour day instead
of a 24 hour one that has to fit in with exogenous cues such as the sun rise. This shows that our
circadian rhythms still function without the EZ's, meaning that there is evidence to show that
there are existing internal body clocks. A study similar to this is Folkard's study on internal body
clocks. He got 12 volunteers to spend three weeks in isolation with no natural light that were told
to go to bed when the alarm clock next to their bed said 23:45pm and set their alarms to wake up
for 7:45am. After a few days of the participants being put in isolation, the clocks in their rooms
were sped up so that the 24 hours were actually passing in 22 hours. He found that only one of the
12 participants kept time with the clock and the rest of them maintained their 24 hour rhythm,
which shows that the endogenous pacemakers were overriding the exogenous zeitgebers. A
problem with this study is that it is extremely small and therefore has low population validity and
so we cannot generalise the findings to the wider population because of individual differences.
An interesting study that gave evidence to show that light is the most significant zeitgeber came
from Miles et al who looked at a man who was born blind and his rhythms. The participant had
always been exposed to exogenous zeitgebers such as alarm clocks and the bird call in the
morning but had never experienced the sun rise zeitgeber. His sleep-wake cycle lasted 24.9 hours
which made it very difficult for him to co-ordinate with the rest of the world and it became so

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Chloe Blunden
difficult for him that he had to use stimulants and sedatives to send him to sleep and wake him up
again. This gives great support for exogenous zeitgebers as it shows they play a significant role in
resetting our rhythms, especially our sleep-wake cycle. To further the support for exogenous
zeitgebers, Campbell shone a light on the back of participants knees, which decreased their
production of melatonin and so they woke up.…read more

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