Discuss the role of endogenous pacemakers and exogenous zeitgebers in circadian rhythms (24 marks)

  • Created by: Marco
  • Created on: 11-04-14 14:29

Discuss the role of endogenous pacemakers and exogenous zeitgebers in circadian rhythms (8+16)

Circadian rhythms are biological cycles that last around 24 hours, such as the sleep/wake cycle and body temperature. This is a free running cycle controlled by an internal mechanism known as the endogenous pacemaker which is an internal biological clock with the role to control the way our biological rhythms behave around a circadian cycle, such as the sleep wake cycle.


One of the main human endogenous pacemakers is the suprachiasmatic nucleus (SCN) which is a small structure inside the hypothalamus which fires neurons on a circadian pattern. The SCN receives information from the optical nerve in the eye, allowing light to be processed to determine our biological rhythms and adapt to those changes. For example, if the sun rises earlier than the day before, morning light will shift the endogenous pacemaker to regulate the body’s circadian rhythm in synch with its environment.
Alternatively, the SCN sends a message at night when there is no light to the pineal gland, causing it to increase the production of melatonin, which induces sleep by preventing the brain’s mechanism to promote wakefulness.


There is much support for the role and existence of the endogenous pacemaker that creates a free running rhythm in the absence of external cues, such as Siffre (1975) who reported a case study of his own experiences in an underground cave for 6 months. Without any external factors such as light or cues to guide him, this demonstrated the existence of an endogenous pacemaker without interference from the outside world. His sleep/wake cycle generally adjusted to a 25 hour cycle, though sometimes dramatically changing to 48 hours.
However, there are issues into this particular study to lack internal validity because although natural light sources were removed, artificial lights from torches and cameras have been shown to reset endogenous rhythms, thus interfering as extraneous variables.
This is exemplified through Czeisler (1999) who altered participants’ circadian rhythms down to 22 hours and up to 28 using dim lighting to alter the melatonin release from the SCN. As a result, the findings do not reflect true isolation and have lower internal validity, so Siffre’s rhythm may not have adjusted as well as in true isolation.
Similarly, Siffre’s study has been criticised for being a case study reflecting his individual


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