Endogenous pacemakers & exogenous zeitgebers

Question 1

What are endogenous pacemakers?

Answer 1

• internal body clocks that regulate many of our biological rhythms such as the sleep wake cycle
• influenced by external environmental factors called exogenous zeitgebers, to control periodic changes.

Question 2

What are animal studies and the SCN?

Answer 2

• mutant hamsters with a 20-hour sleep/wake cycle were bred
• when SCN cells from the foetal tissue of mutant hamsters were transplanted into the brains of normal hamsters, the cycles of the second group defaulted to 20 hours
• both of these studies emphasise the role of the SCN in establishing and maintaining the circadian sleep/wake cycle

Question 3

How does light impact the sleep wake cycle?

Answer 3

• the SCN receives and transfers information about light to the pineal gland
• at night, the pineal gland increases production of melatonin which induces sleep and is inhibited during the day

Question 4

What are social cues?

Answer 4

• in human infants, the initial sleep/wake is pretty much random
• at about 6 weeks of age, the circadian rhythms begin and by about 16 weeks
• the schedules imposed by parents are likely to be a key influence here, including adult determined mealtimes and bedtimes

Question 5

Which study was conducted into the impact of light on the sleep wake cycle?

Answer 5

• 0in an innovate study, Campbell and Murphy demonstrated that light may be detected by skin receptor sites on the body even when the same information is not received by the eyes
• fifteen participants were woken at various times and a light pad was shone on the back of their knees
• the researchers managed to produce a deviation in the participants’ usual sleep/wake cycle of up to 3 hours in some cases
• this suggests that light is a powerful exogenous zeitgebers that need not necessarily rely on the eyes to exert its influence on the brain

Question 6

What are the evaluation points of endogenous pacemakers and exogenous zeitgebers?

Answer 6

• not a complete explanation: changing feeding patterns in mice could alter the circadian rhythms of cells in the liver by up to 12 hours, whilst leaving the rhythm of the SCN unaffected
• ethical issues in animal issues
• a young man, blinds from birth, with a circadian rhythm of 25 hours
• despite exposure to social cues, he had to take sedatives at night and stimulants in the morning. similarly, studies of individuals who live in arctic regions where the sun does not set during the summer months show normal sleep patterns despite the prolonged exposure to light
• methodological issues: that there may have been some limited light exposure to the participants’ eyes which is a major confounding variable

Question 7

How is beyond the master clock a weakness of the sleep/wake cycle?

Answer 7

• research revealed there are numerous circadian rhythms in many organs and cells of the body
• these are called peripheral clocks are highly influenced by the actions of the SCN, they can act independently Daniela et al demonstrated how changing feeding patterns in mice could alter the circadian rhythms of cells in the liver by up to 12 hours, whilst leaving the rhythm of the SCN unaffected
• this suggests that there may be other complex influences on the sleep/wake cycle. Aside from the master clock

Question 8

How are ethics in animal studies a weakness of the sleep/wake cycle?

Answer 8

• on previous spreads we have made reference to the problems involved in generalising findings of the sleep/wake cycle from animal studies to humans
• a more disturbing issue, however particularly in relation to the DeCoursey et al study is the ethics involved in such research
• the animals were returned to their neutral habitat
• whether what we learn from investigations such as these justifies the aversive procedures involved is a matter of debate

Question 9

How is the influence of exogenous zeitgebers being overstated a weakness of endogenous zeitgebers?

Answer 9

• Miles et al recount the story of a young man, blinds from birth, with a circadian rhythm of 24.9 hours
• despite exposure to social cues, his sleep/wake cycle could not adjusted and consequently, he had to take sedatives at night and stimulants in the morning to keep pace with the 24-hour world
• similarly, studies of individuals who live in arctic regions where the sun does not set during the summer months show normal sleep patterns despite the prolonged exposure to light
• both these examples suggest that there are occasions when exogenous zeitgebers may have little bearing on our internal rhythm

Question 10

how are methodological issues in studies a weakness of endogenous pacemakers and exogenous zeitgebers?

Answer 10

• the findings of Campbell et al’s study have yet to be replicated
• other psychologists have been critical of the manner in which the study was conducted and have suggested that there may have been some limited light exposure to the participants’ eyes
• a major confounding variable
• also isolating one exogenous zeitgebers in this way does not give us insight into the many other zeitgebers that influence the sleep/awake cycle, and the extent to which these may interact