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What is an endogenous pacemaker?

an internal "biological clock"


What is an exogenous zeitgeber?

they are external cues from the environment


what is a circadian rhythm?

a cycle that repeats itself every 24 hours e.g. sleep wake cycle


what is an infradian rhythm?

a cycle that repeats itself in a period of over 24 hours e.g. the menstrual cycle


what is an ultradian rhythm?

a cycle that repeats itself more than once in 24 hours


The SCN is the main endogenous pacemaker, explain what it is and what is does in depth

The SCN influences the pineal gland to manufacture melatonin when its dark to induce sleep. The pineal gland is regulated by light which inhibits the production of melatonin. Reptiles and birds do not have an SCN due to them having think skulls and so the light can directly penetrate the pineal gland.


Explain the sleep-wake cycle

The sleep-wake cycle has been thought to be governed by many things, feeling sleepy when its dark and awakening when its light is an option. Another is knowing what time of day it is. However, despite these external cues, there is also an endogenous pacemaker. This is "free running"(works without any external cues). This internal clock doesnt work alone and is influenced by external cues. E.g. light. IN AN EXAM WRITE ABOUT STUDIES SHOWING CR PERSIST DESPITE ISOLATION FROM NATURAL LIGHT AND THAT EXTERNAL CUES ARE IMPORTANT


Research methodology; dim lighting

It was thought that dim lighting would not effect the circadian rhythm and so studies involve participants being isolated from variables such as light, clocks and radios so they didnt have any external cues apart from dim lighting. However, research has shown that by just using dim lighting, a psychologist has been able to lengthen the circadian rhythm from 22-28 hours.


Research support; Siffre

When Siffre spent long period of time in a cave, he had no giveaways to the time of day. He settled into a 25 hr pattern. From this he found 2 key conclusions; there is an inbuilt cue that creates the consistent pattern he endured, and there is a strong influence of external factors that maintained the 24 hour sleep-wake-cycle.


Evaluation of Siffre

This is supported by Wever's WW2 bunker study where participants were put in an underground bunker without any external cues. Their circadian rhythm shifted between 24-25 hours, some were even 29hrs. This shows the cycle operates without any external cues so we must have some sort of internal clock.
HOWEVER, there are individual differences so to say that this can be applied universally is wrong as people do have differences in their cycles. Circadian rhythms should change with each other so the person doesnt suffer from jet lag for example. However, a case study of a lady called Kate showed that when her circadian rhythm changed, he temperate didnt ad so she suffered from jet lag as her rhythms were not in sync. This shows that despite Siffre's rhythm changing to 25hrs, not everyone's will do the same and so you cannot generalise these findings to everyone.
These both are case studies and so again, it is wrong to generalise these findings to everyone.
You can also argue that peoples circadian rhythms vary massively from 13-65hrs. This could be due to the difference in when people's circadian rhythm reach their peak. Some people prefer to wake early and so sleep earlier whereas other prefer to sleep later and wake later. And so to indicate from this research that it can be applied to everyone is wrong and doesn't account for people's individual differences,


What is known as the dominant zeitgeber?



What research is there to support or disprove the idea that light is the dominant zeitgeber?

Miles: studied a man who was blind from birth and had a strong 25hr rhythm. They tried to alter it to 24hr but the rhythm was too strong which stresses the importance of pacemakers and light as an exogenous zeitgeber and the deterministic nature of biological rhythms. However, Luce and Segal found that there are other important zeitgebers such as social customs in places such as the Arctic circle where it is always light in the summer. Light being the dominant zeitgeber is also supported by a study involving students which spent 3 weeks in a lab and were given times to fall asleep and times to wake up. This caused only one of their clocks to shift to 22hrs, this meant that they were able to override their innate rhythm, but because it was only one student, it shows the strength of the 24hr cycle.


Why would there need to be an adaptive reason of the biological rhythm?

DeCoursey: removed the SCN in 30 chipmunks, they then returned to their natural habitat alongside a control group, after 80 days,more of the chipmunks without SCN's had been killed compared to the control group by weasels. This could be because the chipmunks remained awake in their burrows during the night time and so weasels could hear them and killed them. And so there is an adaptive benefit of having an SCN.


Explain core body temperature as being one of the best indicators of the circadian rhythm

Core body temperature is lowest around 4.30am and highest around 6pm, There is a slight trough just after lunch which is why in many countries people have a siesta after lunch.


Evaluate core body temperature and the effects it has on memory and IQ tests

Folkard found that recall of a story book was a lot higher when it was read in the afternoon compared to the morning. This could be due to memory being better when body temperature was higher.
Gupta found that people scored higher on IQ tests during the afternoon


Explain hormones as a circadian rhythm

Cortisol is a hormones produced when we are stressed but it also relates to making us feel alert. This can be used to explain why it is so hard to think clearly in the early hours of the morning; because cortisol is not high enough for alertness. Melatonin and growth hormones also have a circadian rhythm both peaking around midnight.


Evaluate core body temperature being a cause or correlation

There is research to suggest that temperature changes do cause the changes in cognitive performance. Giesbrecht lowered the body temperature of participants and found that cognitive performance was worse.
HOWEVER, Hord and Thompson found no correlation between core temperature and cognitive performance in a field study. And so it is suggested that higher core temperature leads to increased psychological arousal and so this leads to higher cognitive performance.


Real world-application; implications of drug taking

The circadian rhythms affect many important systems in our body e.g digestion. This should thus be taking into account when taking drugs. E.g. taking hormone drugs when the hormone level is low will have hardly any affect compared to when the levels are high. And so the timing of when you take drugs is important in their effectiveness on the body.