endogenous pacemakers and exogenous zeitgebers

Question 1

endogenous pacemakers and the sleep/wake cycle - the SCN

Answer 1

• tiny bundle of nerves in hypothalamus in each hemisphere of brain
• influential in maintaining circadian rhythms
• nerve fibres connected to the eye cross in the optic chiasm on their way to the left and right visual area of the cerebral cortex
• SCN lies just above the optic chiasm, receiving information about light from it
• this continues even when our eyes are closed, enabling the biological clock to adjust to changing levels of daylight when we are asleep

Question 2

endogenous pacemakers and the sleep/wake cycle - animal studies and the SCN

Answer 2

• patricia decoursey destroyed SCN connections in the brains of 30 chipmunks who were then returned to their natural habitats and observed for 80 days
• the sleep/wake cycles disappeared and by the end of the study, a significant proportion had been killed by predators
• martin ralph bred ‘mutant’ hamsters with 20 hour sleep/wake cycles
• when SCN tissue from their brains was implanted into those of normal hamsters, the cycles of the second group defaulted to 20 hours

Question 3

endogenous pacemakers and the sleep/wake cycle - the pineal gland and melatonin

Answer 3

• SCN passes information about day length and light to pineal gland
• this is another endogenous mechanism guiding the sleep/wake cycle
• overnight, the pineal gland increases production of melatonin, this induces sleep and is inhibited in periods of wakefulness

Question 4

exogenous zeitgebers and the sleep/wake cycle - light

Answer 4

• can reset the SCN
• indirect influence on key processes that control functions such as hormone secretion
• Campbell and Murphy showed that light may be detected by skin receptor sites, even when the same information is not received by the eyes
• 15 participants were woken at various times, a light pad was shone on the back of their knees
• managed to produce a deviation in their usual sleep/wake cycles of up to three hours

Question 5

exogenous zeitgebers and the sleep/wake cycle - social cues

Answer 5

• when babies are 6 weeks old, the circadian rhythm begins and by 16 weeks, babies’ rhythms have been entrained by the schedules imposed by parents
• research on jet lag suggests that adapting to local times for sleeping and eating is an effective way of entraining circadian rhythms and beating jet lag

Question 6

evaluation limitation of SCN research - beyond the master clock

Answer 6

• may obscure other body clocks
• numerous circadian rhythms in many organs and cells in the body
• peripheral oscillators are found in the organs, influenced by actions of SCN but also act independently
• Francesca Daniela showed how changing feeding patterns in mice could alter circadian rhythms of cells in the liver by up to 12 hours, while leaving SCN rhythm unaffected
• suggests other complex influences on the sleep/wake cycle

Question 7

evaluation limitation of SCN research - interactionist system

Answer 7

• endogenous pacemakers cannot be studied in isolation
• total isolation studies such as Siffre’s are rare
• in everyday life, pacemakers and zeitgebers interact, and it makes little sense to separate the two for research
• the more that researchers try to isolate the influence of internal pacemakers, the lower the validity

Question 8

evaluation limitation of exogenous zeitgebers - environmental observations

Answer 8

• exogenous zeitgebers do not have the same effect in all environments
• experiences of people who live in places where there is little darkness in summer and little light in winter tell a different story from usual
• people who live in the arctic circle have similar sleep patterns all year round, despite spending six months in total darkness
• suggests that sleep/wake cycle is primarily controlled by endogenous pacemakers that can override environmental changes in light

Question 9

evaluation limitation of exogenous zeitgebers - case study evidence

Answer 9

• evidence challenges the role of exogenous zeitgebers
• Laughton Miles recounts the study of a young man, blind from birth who had an abnormal circadian rhythm of 24.9 hours
• despite exposure to social cues, his sleep/wake cycle could not be adjusted
• suggests that social cues alone are not effective in resetting biological rhythms