Endogenous pacemakers and exogenous zeitgebers

SCN is a primary endogenous pacemaker

The suprachiasmatic nucleus (SCN) is a tiny bundel of nerve cells in the hypothalamus which helps maintain circadian rhythms (e.g. sleep/wake cycle). 

Nerve fibres from the eye cross at the optic chiasm on their way to the visual cortex. The SCN lies just above the optic chiasm and receives information about light from the structure. 

DeCoursey et al (2000) destroyed SCN connections in the brain of 30 chipmunks which were returned to their natural habitat and observed for 80 days. Their sleep/wake cycle disappeared and many were killed by predators. 

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Pineal gland and melatonin are endogenous mechanis

The SCN passes information on day length to the pineal gland which increases the production of melatonin during the night. 

Melatonin is a hormone that induces sleep and is inhibited during periods of wakefulness. It has also been suggested as a causal factor in seasonal affective disorder. 

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Limitation - beyond the master clock

P - One limitation of research into the SCN is that it may obscure other body clocks. 

E - Body clocks (peripheral oscillators) are found in many organs and cells (e.g. lungs, liver, skin and pancreas). They are highly influenced by the actions of the SCN but can act independently.

CA - However, Damiola et al (2000) showed how changing feeding patterns in mice altered circadian rhythms of cells int he liver for up to 12 hours, leaving the rhythm of the SCN unaffected. 

E - On the other hand, it may suggest that there are other complex influences on the sleep/wake cycle, aside from the master clock (SCN). 

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Limitation - Use of animals

P - Another limitation is the use of animals. 

E - There is an issue with generalising the findigns from animal studeis because cognitive factors may be more significant in humans. A more disturbing issue is the ethics of such research - animals were exposed to great harm and potential risk when returned to their natual habitat. 

CA - However, these studies have provided insight into the affects of endogenous pacemakers on the biological clock, meaning that disorders such as SAD (seasonal affective disorder) can be researched to find better treatments. 

E - Nevertheless, whether what we learn from investigations on biological rhythms justifies te aversive procedures involved is a matter of debate. 

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External environmental factors that reset biologic

The German word zeitgeber means 'time-giver'. Resetting biological clocks is a process known as entrainment. 

Without external cues, the free-running biological clock continues to 'tick' in a cyclic pattern. Zeitgebers reset the sleep/wake cycle; an interaction of internal and external factors. 

Light is a key exogenous zeitgeber as it can reset the body's main endogenous pacemaker the SCN and also has an indirect influence on key processes in the body, controlling hormone secretion, blood circulation etc. 

The sleep/wake cycle is fairly random in human newborns but most babies are entrained by about 6 weeks old. Schedules imposed by parents are a key influence, including adult-determined mealtimes and bedtimes. 

Research also shows adapting to local times for eating and sleeping (not responding to one's own feelings of hunger and fatigue) entrains circadian rhythms and tackles jet lag. 

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Campbell and Murphy (1998)

Woke 15 participants at various times and shone light on the backs of their knees - producing a deviation in the sleep/wake cycle of up to 3 hours.. 

Light is a powerful exogenous zeitgeber detected by skin receptor sites and does not necessarily rely on the eyes to influence to SCN. 

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Limitation - the influence may be overstated

P - A futher limitation is the influence of exogenous zeitgebers may be overstated. 

E - Miles et al (1977) note the case of a man blind from birth with a circadian rhythm of 24.9 hours. His sleep/wake cycle could not adjust to social cues so he took sedatives at night and stimulants in the morning to align with the 24 hour world. Similarly, studies of indiviudals who live in arctic regions (where the sun doesn't set during the summer months) show normal sleep patterns despite prolonged exposure to light. 

E - Both these examples suggest there are occassions when exogenous zeitgebers may have little bearing on our internal rhythms. 

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