Endogenous pacemakers and exogenous zeitgebers

• What is the suprachiasmatic nucleus?

  • A tiny bundle of nerve cells located in the hypothalamus in each hemisphere of the brain, as one of the primary endogenous pacemakers
  • Nerve fibres connected to the eye cross in an area called the optic chiasm on their way to the left and right visual areas of the cerebral cortex
  • The SCN lies above the optic chiasm so it directly receives information about light from it, continuing even when our eyes are closed therefore enabling our clocks to adjust to changing patterns of daylight whilst we are asleep
• What research is there into the SCN?

  • DeCoursey at al. (2000) destroyed the SCNs of 30 chipmunks and returned them to their natural habitats and found that their sleep/wake cycles had disappeared - by the end most of them died due to being awake at the same time as predators, being vulnerable to attack
  • Ralph et al. (1990) bred 'mutant' hamsters with a 20 hour sleep/wake cycle and transplanted SCN cells from their foetal tissues into the brains of normal hamsters - the cycles of the second group also defaulted to 20 hours
• What is the role of the pineal gland and melatonin?

  • SCN sends information about light and day length to the pineal gland, which increases production of melatonin during the night and inhibits it during the day
  • Melatonin is a chemical that induces sleep and is inhibited during periods of wakefulness
• What is one limitation of research into endogenous pacemakers?

  • Peripheral oscillators: numerous circadian rhythms in the body can be found in organs like the lungs, pancreas, and skin, etc. that are influenced by the SCN but can also act independently
  • Damiola (2000) found that changing feeding patterns in mice could alter the circadian rhythms in their livers by up to 12 hours leaving the SCN unaffected
  • Suggests there are other complex influences on the sleep/wake cycle
• What is another limitation of research into endogenous pacemakers?

  • Interactionist system: endogenous pacemakers cannot be studied in isolation
  • Totally free-running studies like Siffre's cave study are rare and even his results could've been confounded by the use of artificial light that may have reset his biological clock
  • Everyday life involves the interaction of pacemakers and zeitgebers and it makes little sense to separate the 2 for the purpose of research, suggesting the more researchers attempt to isolate the influences of endogenous pacemakers the lower the validity of the research
• What is another limitation of research into endogenous pacemakers?

  • Ethical issues: animal studies of the sleep/wake cycle like Decoursey et al (2000) show severe ethical problems
  • Decoursey's chipmunks were subject to harm and died as a result of the study, making us question whether it is worth gathering this research when the subjects are at harm and there is a risk of not being able to generalise findings to humans due to differences in influences like mealtimes, working hours, etc.
• What is light as an exogenous zeitgeber?

  • A key zeitgeber as it can reset the SCN and play a role in maintaining the sleep/wake cycle, as well as indirectly influencing key processes in the body that control functions like hormone secretion and blood circulation
• What research is there into light?

  • Campbell and Murphy (1998) woke 15 participants up at various times in the night and a light pad was shone at the back of their knees - this produced a deviation in their usual sleep/wake cycles by up to 3 hours
  • Suggests light is a powerful exogenous zeitgeber that doesn't need to rely on the eyes to exert influence on the brain
• What are social cues as an exogenous zeitgeber?

  • Mealtimes and social activities can act as exogenous zeitgebers that entrain our internal bodily clocks
  • Research on jet lag suggests adapting to local times for eating and sleeping rather than responding to our biological drives of hunger and fatigue is an effective way of entraining circadian rhythms
• What is one limitation of research into exogenous zeitgebers?

  • Environmental observations: some people live in places where there is very little darkness in summer and little light in winter will have different experiences
  • People who live in the Arctic Circle have similar sleep patterns all year round despite spending 6 months in total darkness
  • Suggests sleep/wake cycle is primary controlled by endogenous pacemakers that override environmental changes
• What is another limitation of research into exogenous zeitgebers?

  • Case study evidence: Miles et al. (1977) studied a young man who was blind from birth with an abnormal circadian rhythm of 24.9 hours
  • Despite exposure to social cues like regular mealtimes his sleep/wake cycle still would not be adjusted, suggesting social cues alone are not effective in resetting the biological rhythm