9. Endogenous Pace Makers + Exogenous Zeitgebers (BRs - CRs)

• 2c. Biological Rhythms - Circadian Rhythms

  S/W Cycle:

  • synchronised by endogenous pacemakers, set pace for body cycles.
  • master pacemaker = suprachiasmatic nucleus.
• 2d. Biological Rhythms - Circadian Rhythms

  Suprachiasmatic Nucleus (SCN):

  • light enters eyes; light-sensitive cells act as detectors.
  • send message about environmental light levels to SCN.
  • info then used coordinate activity of s/w cycle.
  • explains why sleep when dark, wake when light.
• 3. Biological Rhythms - Circadian Rhythms

  Animal Studies - Decoursey et al. (2000):

  • destroyed SCN connections in brain of 30 chipmunks; returned to natural habitat + observed for 80 days.
  • S/W cycle disappeared; many killed by predators :(
• 3a. Biological Rhythms - Circadian Rhythms

  Animal Studies - Ralph et al. (1990):

  • bred ‘mutant’ hamsters w/ 20hr s/w cycle.
  • SCN cells from tissues of ‘mutant’ hamsters transplanted into normal hamsters.
  • cycle of 2nd group to 20hrs.
• 4. Biological Rhythms - Circadian Rhythms

  Pineal Gland + Melatonin:

  • SCN passes info on day length + light it receives to pineal gland.
  • during night, pineal gland increase melatonin.
  • melatonin encourages sleep; peak production during hrs of darkness, reducing through hrs of light to encourage alertness.
• 5. Biological Rhythms - Circadian Rhythms

  S/W Cycle:

  • also under homeostatic control (another endogenous pacemaker).
  • homeostasis increases our need for sleep based on amount of energy using while awake.
  • happens regardless of whether dark/light.
  • most people who wake in morning, sleep in evening - homeostatic clock + circadian rhythm are in sync.
• 6. Biological Rhythms - Circadian Rhythms

  Core Body Temp:

  • another indicator of circadian rhythm.
  • sleep occurs when core temp, begins drop (reaches lowest of around 36 degrees at 4:30am) + rises through waking hrs to about 38 degrees.
  • small drop in body temp. occurs in late evening.
• 6a. Biological Rhythms - Circadian Rhythms

  Campbell + Murphy’s Light Pad Study:

  • findings suggest light is such powerful exogenous zeitgeber doesn‘t necessarily need enter eyes to affect sleep wake cycle - can affect biological rhythms.
  • during study, woke 15 participants at random intervals during night to shine light pad on back of knees.
  • managed create deviation up to 3hrs in s/w cycle.
• 6b. Biological Rhythms - Circadian Rhythms

  Other factors affecting S/W Cycle:

  • social cues e.g. meal time, bedtime routines.
  • on average, takes baby up to 16wks for s/w cycle to entrained (natural occurrence) - schedules imposed by parents likely be key influence here, including adult-determined mealtimes/bedtimes.
  • seen effective in beating jet lag.
• Biological Rhythms - Circadian Rhythms (Evaluation)

  Limitation (Endogenous P + Exogenous Z):

  P - Decoursey et al. (2000); ethical issues.

  E - harm in study + risk when returned to natural habitat.

  L - debate over what learned in study justifiable.
• Biological Rhythms - Circadian Rhythms (Evaluation)

  Limitation (EP + EZ):

  P - man, blind from birth, w/ CR of 24.9hrs.

  E - exposure to social cues, s/w cycle not be adjusted so had to take sedatives (night) + stimulants (morning) to keep pace w/ 24hr world.

  L - not correct for all individual people.
• Biological Rhythms - CRs (Evaluation)

  Limitation (EP + EZ):

  P - Campbell + Murhpy not replicated.

  E - critical of manner study conducted in, suggested some light exposure to participants eyes - cofounding variable.

  L - isolating 1 exogenous z in this way not give inshight to other zeitgebers influencing s/w cycle.