endogenous pacemakers + exogenous zeitgebers

• endogenous pacemakers:

  • SCN = tiny bundle of nerves located in hypothalamus in each hemisphere
  • lies above optic chiasm - receives info about light, continues when eyes are closed
• pineal gland + melatonin:

  • SCN passes info on day length + light to pineal gland
  • during night - pineal gland increase production of melatonin
  • increases in evening, peaks at middle of night, decreases in morning
• DeCoursey et al:

  • destroyed SCN connections in 30 chipmunks, returned to natural habitats, observed for 80 days
  • sleep wake cycle dissapeared + large proportion were killed by predators
• Ralph et al:

  • bred mutant hamster with 20 hour sleep wake cycle
  • SCN cells from foetal tissues were transplanted into normal hamsters, cycles defaulted to 20 hours
• exogenous zeitgebers:

  • reset biological clock through entrainment = process of making something have same pattern as something else
  • absence of external cues = biological clock continues to 'tick' in cyclical pattern
  • sleeping/wakefulness determined by interaction of external/internal cues
• light:

  • main exogenous zeitgebers in humans
  • maintains sleep wake cycle, indirect influence on key processes (hormone secretion + blood circulation)
• Campbell + Murphy:

  • light may be detected by skin receptor sites
  • 15 people woken at different times, light pad shone on back of knees - deviation of sleep wake cycle up to 3 hours
  • light = powerful exogeneous zeitgeber + doesnt rely on eyes to reflect influence
• social cues:

  • circadian rhythms begin at 6 weeks
  • by 16 weeks = rhythms are entrained by schedules imposed by parents e.g mealtimes
• jet lag research:

  • adapting to local times for eating/sleeping instead of responding to own feelings = more effective way of entraining rhythm + beating jet lag
• EVALUATION: interactionist system (cant be studied in isolation)

  • siffres study is rare -doesnt prevent entrainment due to artificial light, biological clock reset everytime lamp turned on
  • everyday life = pacemakers and zeitgebers interact, makes little sense to separate 2 for research
  • more researchers attempt to isolate influence of pacmaker - decrease validity
• EVALUATION: ethics in animal studies

  • arguments that studies can be generalised as mammalian brains have similar structure - critics disagree with research due to ethics
  • DeCoursey et al - chipmunks subject to invasive procedure + exposed to risks and death
  • what we learn may not justify unethical procedures
• EVALUATION: role of EZs is exaggerated

  • Miles et al - young man blind from birth , 24.9 circadian rhythm. exposure to social cues - didnt adjust rhythm, had to take sedatives + stimulants to keep pace
  • arctic regions - normal patterns despite prolonged exposure to light in summer
  • there are occasions when EZs have little bearing on rhythms