Biological Rhythms: Circadian Rhythms from the module of Biopsychology has been summarised from the AQA psychology textbook for A LEVEL published by Illuminate Publishing.
All living organisms are subject to biological rhythms and these exert an important influence on the way
in which body systems behave. All biological rhythms are governed by the body’s internal biological
clocks (endogenous pacemakers) and external changes in the environment (exogenous zeitgebers).
Ultradian rhythms- occur many times during the day.
Infraradian rhythms- take longer than a day to complete.
Circannual rhythms- much longer.
Circadian rhythms- last for around 24 hours. Sleep/wake cycle and core body temperature.
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, The sleep/wake cycle
An important exogenous zeitgeber on our cycle is that we feel drowsy when it’s night time and alert
during the day. Free-running is when the biological clock is left to its own devices without the influence
of external stimuli.
Siffre’s cave study (pronounced “Seef”)
He is a self-styled caveman who has spent several extended periods underground to study the effects on
his own biological rhythms. He was deprived of exposure to natural light and sound, he re-surfaced in
mid-September 1962 after 2 months, believing it was mid-August. A decade later he did this again for 6
months in a different cave.
In each case, his “free-running” biological rhythm settled down to one that was just beyond 2 hours
(around 25 hours) though he did continue to fall asleep and wake up on a regular schedule.
Other Research
Similar results were recorded by Jurgon Aschoff and Rutger Wever (1976) who convinced a group of
participants to spend 4 weeks in a WWII bunker deprived on natural light. All but one participant (whose
sleep/wake cycle extended to 29 hours) displayed a circadian rhythm between 24-25 hours. Both studies
suggest that our “natural” sleep/wake cycle may be slightly longer than 24 hours but is entertained by
exogeneous zeitgebers associated with the 24-hour day.
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