Endogenous and exogenous

• What are the weaknesses of research into the importance of endogenous pacemakers and exogenous zeitgebers?


  ethical breach of BPS guidelines with animal studies as the chipmunks experienced physical harm and eventual death so a cost benefit analysis should be conducted. Furthermore this research suffers generalisability issues as chipmunks and humans have different physiology's meaning we cannot be sure these results would replicate amongst humans.

  
  

  Miles et al found that sometimes exogenous zeitgebers and endogenous pacemakers have no effect on circadian rhythms through the case study of a man with a sleep wake cycle f 24.9 hours that could not be changed no matter what stimulants or sedatives were used, suggesting the role of these factors may be overestimated and not provide a comprehensive explanation for circadian rhythms.
• What research supports the importance of exogenous zeitgebers in the sleep wake cycle?

  Supporting evidence of a change in endogenous pacemakers and exogenous zeitgebers having an effect on circadian rhythms comes from Siffre in 1973 who found that when he returned from an underground stay deprived of light, he believed the date to be a month earlier than it was. This suggests that his 24-hour sleep-wake cycle was increased by the lack of external cues.

  
  

  Additionally, Aschoff and Weaver (1967) deprived 55 participants of natural light for 4 weeks and found their sleep wake cycles ranged from 23.9 to 50 hours suggesting that exogenous zeitgebers conform the rhythm to 24 hours.

  
  

  Furthermore, Campbell and Murphy produced deviations of 3 hours in participant's sleep-wake cycle by shining light onto pads on the back of their knees, showing that light does not always need to be detected by the eyes in order to entrain biological rhythms.
• What supports the importance of the SCN in the sleep wake cycle?

  DeCoursey et al in 2000 who surgically lesioned the SCNs of 30 chipmunks and compared there circadian rhythms in their natural habitats to untouched controls. They found the majority of the experimental group had been killed within 80 days due to nocturnal movement. Suggesting the SCN is influential in maintaining circadian rhythms that aid survival.
• What can a change in either endogenous pacemakers or exogenous zeitgebers cause?

  endogenous pacemakers and exogenous zeitgebers for example melatonin levels, working night shifts, and jet lag, can cause desynchronisation of biological rhythms.
• Give an example of an exogenous zeitgeber.

  light levels in the sleep wake cycle and social cues in the digestive cycle (through set societal meal times)
• What are exogenous zeitgebers?

  Exogenous zeitgebers are environmental events which reset the body clock.
• What is the most important endogenous pacemaker?

  The suprachiasmatic nucleus (SCN) receives information about day light and length from the eyes, and processes this to then trigger different rates of release of melatonin from the pineal gland.

  
  

  Low light exposure increases melatonin which decreases the production of serotonin, creating feelings of sleepiness during the night time. High light exposure causes less melatonin to be secreted, increasing the production of serotonin, creating feelings of wakefulness.
• What are endogenous pacemakers?

  Endogenous pacemakers are internal mechanisms that govern biological rhythms. They can be affected by environmental factors.
• What two main factors influence biological rhythms?

  endogenous pacemakers (internal factors), the body's biological clocks, and exogenous zeitgebers (external factors), which are changes in the environment.