Exogenous zeitgebers and endogenous pacemakers

• Endogenous pacemakers

  Suprachiasmatic nucleus (SCN) is a tiny bundle of cells located in the hypothalamus of each hemisphere

  • It's influential in maintaining circadian rhythms like the sleep/wake cycle.
  • Nerve fibres connected to the eye cross in an area called the optic chiasm on their way to the left and right visual area of the cerebral cortex.
  • The SCN lies above the optic nerve.
  • It receives information about light directly from this structure.
  • This also continues when our eyes are closed.
• Animal studies and the SCN

  DeCoursey destroyed the SCN connections in the brains of 30 chipmunks who were then returned to their natural habitat, and observed for 80 days.

  • The sleep/wake cycle of chipmunks disappeared, by the end of the study a significant proportion had been killed by predators.
  • Ralph bred 'mutant' hamsters with a 20 hour sleep/wake cycle.
  • When the SCN cells from foetal tissue of mutant hamsters were transplanted into brains of normal hamsters, the cycles of 2nd group defaulted to 20 hours.
• The pineal gland and melatonin

  The SCN passes the information on day length and light that it receives to the pineal gland. This is another endogenous mechanism guiding the sleep/wake cycle.

  During the night the pineal gland increases production of melatonin- a chemical that induces sleep and is inhibited during periods of wakefulness

  Melatonin has also been suggested as a cause factor in seasonal affective disorder.
• Evaluation- beyond the clock

  Limitation= may obscure other body clocks

  • Research has revealed that there are numerous circadian rhythms in many organs cells cells in the body.
  • These peripheral oscillators are found in the organs including the lungs, pancreas and skin.
  • They are influenced by the actions of the SCN, but also act independently
  • Damiola demonstrated how changing feeding patterns in mice could alter the circadian rhythms of cells in the liver by up to 12 hours, whilst leaving rhythm of SCN unaffected.
• Evaluation- interactionist system

  Limitation= endogenous pacemakers cant be studied in isolation.

  • Total isolation studies like Siffre's cave study are very rare.
  • Siffre made use of artificial light which could have reset his biological clock every time he turned the lamp on.
  • In everyday life, pacemakers and zeitgebers interact, and it may make little sense to separate the two for the purpose of research.
• Exogenous zeitgebers- light

  It can reset the bodys main endogenous pacemaker, the SCN, and thus play a role in the maintenance of sleep/wake cycle.

  • Direct influence on processes that control functions like hormone secretion and blood circulation.
• Campbell and Murphy study 


  Demonstrated that light may be detected by skin receptor sites on the body even when the same information is not received by the eyes.

  15 participants were woken at various times and a light pad was shone on the back of their knees.

  Managed to produce a deviation in participants usual sleep/wake cycle of up to 3 hours.

  This suggests that light is a powerful exogenous zeitgeber that need not necessarily rely on the eyes to exert its influence on the brain.
• Social cues

  Babies are seldom on some sleep/wake cycle as rest of the family, a newborns initial sleep/wake cycle is random.

  At around 6 weeks old, circadian rhythms begin, by 16 weeks babies rhythms have been entrained by the schedules imposed by parents.

  Research on jet lag suggests that adapting to local times for eating and sleeping is an effective way of entraining circadian rhythms
• Evaluation- environmental observations

  Limitation= exogenous zeitgebers don't have same effect in all environments.

  The experience of people who live in places where there is little darkness in summer and little light in winter.

  People who live within the Artic circle have similar sleep patterns all year round, despite spending months in darkness.

  Suggests the sleep/wake cycle is primarily controlled by endogenous pacemakers can override environmental changes in light.
• Evaluation- case study evidence

  Limitation= evidence challenges the role of exogenous zeitgebers.

  Miles recount the study of a young man, blind from birth, who had abnormal circadian rhythm of 24.9 hours.

  Despite exposure to social cues eg: regular mealtimes, his sleep/wake cycle couldn't be adjusted.

  suggests that social cues aren't effective in resetting the biological rhythm.