Bodily Rhythms

The role of Endogenous pacemakers and Exogenous Zeitbeigers

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Circadian: within 24 hours, e.g. sleep-wake cycle

Infradian: longer than 24 hours, e.g. menstrual cycle

Ultradian: less than 24 hours, e.g. NREMREM sleep

Endogenous Pacemaker: An internal clock that sets a periodicity or rhythm.  This biological mechanism may be set 24 hours, annually or any other length. 

Exogenous Zeitbeger: Environmental influence that may affect cycle. 

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Circadian Rhythms: Endogenous Pacemakers

Evidence to suggest they exist:

  • Siffre (1975), 11 periods of isolation.  In one incident, spent 6 months with no access to normal light.  Free running biological clock.  Initially, no clear pattern in his sleep-wake cycle.  However, over time, he developed a cycle of 24 hrs 30 mins.  When he came out it was his 151st day when it was really 179th.

+ Similar results over 11 cases - consistent, reliable.  +  Internal Validity - no external cues  - Internal Validity; other factors affecting results such as humidity, temperature - External Validity - Applicable to others?

  • Wever reviewed several studies of ppts who spent 2 weeks - 4 months in bunkers/isolation suites.  Sleep-wake cycle settled to 25 hours. 

+Internal Validity - no external cues.  + Several ppts, consistency.  - Meta-analysis- don't know procedures, implications of external validity where?who?  - Artificial environment, extrapolation

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Circadian Rhythms: Endogenous Pacemakers (cont.)

What is the endogenous pacemaker?:  The SCN (Suprachiasmatic Nucleus) is a group of cells located in the hypothalamus just above the optic chiasm so that it an receive input directly from the eye; our rhythm can therefore be reset by the amount of light.  Messages then sent to the pineal gland which releases melatonin when light levels are low. 

Research Evidence for the SCN:

  • Morgan (1995) removed the SCN from hamsters and found their circadian rhythms disappeared. 

+Lab experiment - highly objective and scientific.  + More than one hamster - reliable, consistent.  - Artificial environment, highly controlled.  - Animals, extrapolation. 

  • Silver et al: showed that transplanted SCNs could restore the circadian rhythms of animals whose own SCN had been ablated.

+Lab experiment, highly objective and scientific.  + More than one, consistency  - Non-human animals, extrapolation.  - Artificial.  - Unethical

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Circadian Rhythms: Endogenous Pacemakers (cont.)

The Role of Melatonin:

  • Schohat et al measured melatonin production in 6 male participants over 29hr period and found a correlation between sleep propensity (time of day) and melatonin levels. 

+ Lab setting.  + 6 males, consistent

- Androcentric, small, possibly ethnocentric.  - Artificial - ecological v.  - Correlation, causation.  - Short period of time

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Circadian Rhythms: Exogenous Zeitbegers (External

Research Evidence:

  • Campbell and Murphy applied lights to the back of human participants knees.  They were able to shift the circadian rhythms, suggesting that light is an important external cue and isn't just received by the eye. +Lap expt.  Causation can be established.  Artificial
  • Segal documented sleep-wake cycles of people living in the Arctic circle and found that they still slept for 7 hours despite sun not setting in the summer.  Other external factors such as social customs may take over. + Real people, natural habitat.  - Ethnocentric- people from Arctic may have evolved differently.  - Causation. 
  • Miles et al documented problems of a young man blind from birth.  He had a strong 24.9 hour rhythm until the use of sedatives, suggesting that there is an internal body clock but we do need the help of external cues.   +Real person +Internal validity - no external cues.  -Case study, generalisability.  - Not controlled, Internal validity.  
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Infradian Rhythms: Endogenous Pacemakers:

The menstrual cycle:  is governed by the female hormones oestrogen and progesterone which cause the lining of the womb to become engauged with blood and one egg to ripen and release.  This takes on average, 28 days. 

Research Evidence: Reinberg (1967) documented that when a women spent 3 months in a cave, her sleep-wake cycle lengthened and her menstrual cycle shortened. 

Circadian rhythms/ the SCN can influence Infradian rhythms. 

+ No external Cues.  - Living in such conditions may cause changes.  Artificial

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Inradian Rhythms: Exogenous Zeitbegers:

It has been a common observation that women who spend time together, such as boarding school girls and nuns, appear to have synchronised menstrual cycles.  It has been suggested that biochemical substances called pheromones carry messages from one individual to another.

Research Evidence: Russel et al (1980) found that when the pheromones of a female donor were applied to a group of women over 5 months, their menstrual cycles synchronised to within a day of the donor.  

+Control group.  +Lab experiment.  - Artificial, procedures don't necessarily match real life - involved applying sweat from armpit to upper lip.  - Sample of 5 women. 

Pheromones, external factors, have power to reset cycle. 

Explanation: The synchronisation was an evolutionary advantage; women could breastfeed at the same time - if one women died, child could be shared. 

Women who work with men have shorter menstrual cycles - more opportunities to become pregnant.  Suggestive of male pheromones.  Would otherwise be disadvantageous, drain of resources.


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