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Circadian Rhythm summary sheet
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Circadian RhythmsAll living organisms have biological rhythms which impact on our body’s systems.
Define the following key terms (use your own words)
Biological rhythms:
Distinct patterns of changes in body activity that conforms to cyclical time periods.
Biological rhythms are influenced by internal body clocks (endogenous pacemakers) as
well as external changes to the environment (exogenous zeitgebers)
Circadian rhythms:
A type of biological rhythm, subject to a 24-hour cycle, which regulates a number of body
processes such as sleep/wake cycle and changes in core body temperature
Exogenous
zeitgebers: External changes to the environment
Endogenous
pacemakers Internal body clocks
The sleep/wake cycle is a key example of circadian rhythms- we feel drowsy and sleepy at nighttime and
alert during the day, these are key examples of endogenous pacemakers.
Many studies have been conducted to assess the sleep/wake cycle to see what would happen if the
biological clock had no exposure to external factors.
Key research: Siffre’s cave study
Summarise the research and findings using 4 bullet points.
• Michael Siffre was a self-taught caveman who used to stay underground for long periods in order to
study the effects of his own biological rhythms
• He stayed underground for around 2 months, with a lack of sunlight and sound but enough food and
drink
• A decade later he performed a similar study, this time 6 months in a Texas cave
• In each case, his ‘free-running’ biological rhythm settled down to one that was beyond usual 24 hours- 25
instead, although he woke up and fell asleep on a regular schedule
, Other research
(use either Aschoff and Wever or Folkard)
Similar results were recorded by Aschoff and Wever, who convinced a group of participants to spend four weeks in a WWII
bunker deprived of natural light. All but one of the participants (whose sleep/wake cycle extended to 29 hours) displayed a
circadian cycle between 24 and 25 hours.
Both Siffre and the bunker study suggest that the ‘natural’ sleep/wake cycle may be slightly longer than24 hours but that it
is entrained by exogenous zeitgebers associated with our 24- hours day (such as the number of daylight hours, typical meal
times, etc.
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