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Discuss the role of endogenous pacemakers and exogenous zeitgebers in circadian rhythms.
We have an internal body clock the lasts the duration of a day (24 hours), which is therefore able to
control the circadian rhythm. As the biological body clock is controlled inside it is known as
endogenous, these are called endogenous pacemakers. So how do these work on their own to
affect the sleep-wake cycle and why do we need external factors to also help control our circadian
There have been many studies into endogenous pacemakers to see if our sleep-wake cycle
(circadian rhythms) will still last 24hours without any exogenous zeitgebers(external factors)
influencing this internal body clock.
In order to study internal pacemakers you need to take away all possible external factors. Folkard
achieved this by doing a study where 6 students spent a month isolated from these external cues. He
recorded data and findings from the children by monitoring temperature, activity level and mood
every 2 hours. With the absence of external factors, biological findings proved the existence of
internal body clocks which control sleep/waking temperature etc. This study was carried out well.
Folkard chose a good length of time so the students could adjust and the conditions were as realistic
as possible as the students were given tasks that they'd normally do e.g. playing bagpipes, the
closest you could get to a field study without external factors. Therefore results found are quite
reliable but although Folkard found evidence of endogenous pace makers, results may be hard to
generalise because the sample size was fairly small and all of similar age and you have to be careful
of confounding variables affecting results as the students knew each other and could be
synchronised with each other to when they should eat together as a group and sleep.
Siffre cave study.
Similarly another study also took place with a participant isolated from external factors to prove the
existence of endogenous pacemakers without interference of the outside world. Siffre spent 6
months in a cave where there was obviously no light or sound therefore he could not tell the time of
day. At the end of the study they found his physiology and behaviour was still cyclical, but his day was
an hour longer (25 hours). Although this study was for a good length of time, as the Folkard study
supports this, there are many evaluative problems with this study. Firstly there was only one
participant in this study, so results are not easily generalised and you have to bear in mind that there
are individual differences for example he is a man of a particular age so results could be said to be
androcentric. The main problem with this study is that it is artificial; no one would realistically spend 6
months in a cave. Other factors such as loneliness and temperature can affect in this study. But we still
need to make note than even though there may be some issues with the study, Siffre still had a daily
cycle, he knew when to wake, when to eat and when it was time to go to sleep, which shows that
there must be an internal body clock that controls our sleep-wake cycle.
There is also conflicting research by Czeisler as he believed that Siffre's Study and others which found
similar results had a big methodological flaw. He challenged these studies as he believed bright
artificial lights `reset' the body clock. So he carried out an experiment using low subdued light to
affect the study as little as possible and attempted to make the cycle 28 hours. They found by
monitoring body chemistry and temperature, that the body clock is actually 24 hours and 11 mins and
not 25 found in many previous studies.
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Czeisler's research shows that there are external variables such as light and temperature that affect
our sleep-wake cycle. So if our body clock is reset by bright light how does our body do this?
One of our internal pacemakers is in the SN. It receives information from the eye about the light and
dark and incorporates this in our rhythms with the natural 24hr daily cycle outside. The Hamster study
by Morgan 1995 shows just how the SN can severely change our cycle.…read more
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controlling our sleep-wake cycle as our pineal gland is sensitive to light and secretes melatonin and
helps us keep in time with the outside world of being typically awake during light and asleep at night.…read more