endogenous pacemakers and exogenous zeitgebers

• endogenous pacemakers - 


  internal body clocks that regulate many of our biological rhythms

  such as influence of suprachiasmatic nucleus on the sleep/wake cycle
• the suprachiasmatic nucleus -

  • tiny bundle of nerves located in hypothalamus in each hemisphere of the brain
  • one of primary endogenous pacemakers - influential in maintaining circadian rhythms
  • 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
  • SCN lies above optic chiasm - receives information about light directly from this structure - continues even when eyes are closed - enables bio clock to adjust to changing patterns of daylight whilst asleep
• animal studies and the SCN -

  • DeCoursey et al (2000) destroyed the SCN connections in the brains of 30 chipmunks - returned to natural habitat and observed for 80 days - sleep/wake cycle of chipmunks disappeared and a significant portion had been killed by predators (presumably cause they were awake, active and vulnerable to attack when they should've been asleep)
  • Ralph et al (1990) bred 'mutant' hamsters with 20 hour sleep/wake cycle - SCN cells from foetal tissue of mutant hamsters were transplanted into brains of normal hamsters - cycles of 2nd group defaulted to 20hrs
• pineal gland and melatonin -

  • SCN passes info on day length and light it receives to the pineal gland - another endogenous mechanism guiding the sleep/wake cycle
  • during the night the pineal gland increases production of melatonin - chemical that induces sleep and is inhibited during periods of wakefulness
  • melatonin also suggested as a causal factor in SAD
• exogenous zeitgebers - 


  external factors that affect or entrain our biological rhythms

  such as influence of light on sleep/wake cycle
• exogenous zeitgebers and the sleep/wake cycle -

  in the absence of external cues, the free-running biological clock that controls the sleep/wake cycle continues to 'tick' in a distinct cyclical pattern

  free-running cycle is them 'brought into line' by environmental cues so there's an interaction of internal and external factors
• exogenous zeitgebers: light -

  • maintenance of sleep/wake cycle + indirect influence on functions such as hormone secretion and blood circulation
  • Campbell and Murphy (1998) demonstrated light may be detected by skin receptor sites even when same info is not received by the eyes
  • 15 participants woken at various times + a light pad was shone on back of their knees - researchers managed to produce a deviation in participants usual sleep/wake cycle of up to 3 hours
  • suggests light is a powerful exogenous zeitgeber that need not necessarily rely on the eyes to exert its influence on the brain
• exogenous zeitgebers: social cues -

  • newborn babies initial sleep/wake cycle is pretty much random
  • at 6 weeks of age the circadian rhythms begin and by 16 weeks babies rhythms have been entrained by the schedules imposed by parents including adult determined mealtimes and bedtimes
  • research on jet lag suggests adapting to local times for eating and sleeping is an effective way of entraining circadian rhythms and beating jet lag when travelling long distances
• evaluation of endogenous pacemakers: beyond the master clock (limitation) -

  • P: may obscure other body clocks
  • E: research revealed numerous circadian rhythms in many organs and cells in the body - peripheral oscillators are found in the organs including the lungs, pancreas and skin - influenced by SCN but also act independently
  • E: Damiola et al (2000) demonstrated how changing feeding patterns in mice could alter circadian rhythm of liver cells up to 12 hours whilst leaving rhythm of SCN unaffected
  • L: suggests other complex influences on the sleep/wake cycle
• evaluation of endogenous pacemakers: interactionist system (limitation) -

  • P: can't be studied in isolation
  • E: total isolation studies such as Siffre's cave study are extremely rare - also made use of artificial light which could've reset his biological clock
  • E: in everyday life pacemakers and zeitgebers interact and may make little sense to separate the 2 for purpose of research
  • L: suggests the more attempt to isolate the influence of internal pacemakers, the lower the validity of the research
• evaluation of exogenous zeitgebers: environmental observations (limitation) -

  • P: doesn't have the same effect in all environments
  • E: experience of people who live in places where there is very little darkness in summer and very little light in winter tell a different story from the usual narrative
  • E: eg people who live in arctic circle have similar sleep patterns all year round, despite spending 6 months in almost total darkness
  • L: suggests the sleep/wake cycle is primarily controlled by endogenous pacemakers that can override environmental changes in light
• evaluation of exogenous zeitgebers: case study evidence (limitation) -

  • P: evidence challenges the role of exogenous zeitgebers
  • E: Miles et al (1977) recount the study of a young man, blind from birth - had abnormal circadian rhythm of 24.9 hours - despite exposure to social cues such as regular mealtimes, his sleep/wake cycle couldn't be adjusted
  • L: suggests social cues alone are not effective in resetting the biological rhythm