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A biological rhythm is a cyclical variation over a period of time in physiological or
psychological processes. There are three main biological rhythms which are Circadian,
Infradian and Ultradian. These rhythms are controlled by internal body clocks called
endogenous pacemakers and external cues from the environment called Exogenous
Circadian Rhythms are biological rhythms that occur ONCE every 24 hours; examples to
explain this rhythm are the sleep- wake cycle and core body temperature.
The sleep wake cycle is governed by an internal body clock called endogenous pacemakers,
however the internal body clock does not work alone, exogenous zeitgebers which are
external cues from the environment for example daylight help to adjust the internal clock to
the environment in which an individual lives in. A study has been done to provide evidence of
this done by Siffre who wanted to investigate what would happen to the circadian rhythm if
a person was cut off from all zeitgebers, therefore relying just on endogenous pacemakers.
He stayed for 7 months in a cave, with no clue to what time of day it was. He eventually
settled into a regular pattern of sleeping and waking with a 24.9 rhythm, so he was waking
up an hour late. To conclude, this study shows that we do have internal mechanisms to
regulate the sleep wake cycle, but there is a shift to 25 hours if there are no zeitgebers
present to reset it.
Core body temperate is one of the best indictors of the circadian rhythm. It is lowest at
around 4:30 am (36'c) and highest at around 6pm (38'c). There is a slight trough after lunch,
not due to the effects of having lunch, as the dip occurs even if you haven't eaten. In many
countries this is the reason why individual have a `siesta' because of the dip in their body
temperature which is known as a bi-daily rhythm.
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There is evidence support for core body temperature, that it is linked to cognitive abilities,
which is supported by Folkhard et al who looked at learning abilities in 12 to 13 years old. The
children had stories read to them at either 9am or 3pm; a week later the afternoon group
who would have had higher core body temperature showed superior recall and
comprehension. This suggests that long term recall is best when body temperature is at its