9. Pacemakers and Zeitgebers

• Suprachiasmatic nucleus

  • Primary endogenous pacemaker and maintains circadian rhythms
  • Tiny bundle of nerve cells located in the hypothalamus in each hemisphere of the brain
  • Lies above optic chiasm and receives information about light
  • Continues when our eyes are closed, enabling the biological clock to adjust to daylight's changing patterns whilst we are asleep
• Animal studies and the SCN

  • Decoursey et al. destroyed SCN connections in the brains of 30 chipmunks who returned to their natural habitats and observed for 80 days
  • Sleep/wake cycles had disappeared and a significant proportion of them had been killed by predators
  • Ralph et al. bred mutant hamsters with a 20 hour sleep/wake cycle
  • SCN cells from the foetal tissue were transplanted into the brains of normal hamsters, their cycles were defaulted to 20 hours
• The pineal gland and melatonin

  • SCN passes info on day length and light to the pineal gland
  • This is another endogenous mechanism guiding the sleep/wake cycle
  • During the night, the pineal gland increases the production of melatonin which induces sleep
• Light as an exogenous zeitgeber

  • Resets SCN
  • Indirect influence on key processes in the body that control functions such as hormone secretion and blood circulation
  • Campbell and Murphy - demonstrated light may be detected by skin receptor sites on the body even when the same info is not received by the eyes
  • 15 pps woken at different times and a light pad was shone at the back of their knees
  • Deviation of usual sleep/wake cycle of up to 3 hours produced
  • Suggests light is a powerful EZ and reliance on the eyes is not always needed to exert influence on the brain
• Social cues as an exogenous zeitgeber

  • Newborn babies initial sleep/wake cycle is random
  • Circadian rhythms begin at about 6 weeks old and by 16 weeks they have been entrained by schedules imposed by parents including meal times and bed times
  • Jet lag research suggests that adapting to local times for eating and sleeping is an effective way of entraining circadian rhythms and beating jet lag when travelling long distances
• AO3 - Strength of research support

  • Both Decoursey et al. and Ralph et al.’s research demonstrate the importance of the SCN in regulating the sleep/wake cycle
  • Decoursey’s chipmunks - lack of an SCN meant the pineal gland did not receive the signal to produce melatonin, which completely destroyed their sleep/wake cycle
  • It appears that the effect of the SCN is far more than any exogenous zeitgeber might have (for example Siffre’s sleep/wake cycle only being altered by an hour vs a lack of sleep completely in Decoursey chipmunks)
  • Using animals as pps - high internal validity and more control
• AO3 for EPs - Damiola research

  • Research has revealed that there are numerous circadian rhythms in many organs and 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.
  • Francesca Damiola et al. (2000) demonstrated how changing feeding patterns in mice could alter the circadian rhythms of cells in the liver by up to 12 hours, whilst leaving the rhythm of the SCN unaffected
  • Reductionist approach could develop with too much focus on SCN
  • Social cues (jet lag)
• AO3 for EPs - Interactionist approach and reductionism

  • Total isolation studies, such as Siffre's cave study are extremely rare
  • This study was reductionist as it only focused on