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Circadian Rhythms
1. THE MAIN Circadian rhythm is the sleep-wake cycle = lasts 24 hours.
Endogenous pacemakers (internal clocks) and exogenous zeitgebers (external time givers) are
both involved.
For example light, as an exogenous zeitgeber tells us when to go to bed and when
to wake up. The presence/absence of sunlight provided us with information about
what time it is.
Endogenous pacemakers may be just as important...
Research has found that our internal clock is free running and may actually work
independently of external cues.…read more

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Folkard et al - read stories to children aged 12-13 years old at either 9am or 3pm
= the afternoon group (highest core body temp) retained more information - 8% more than the
morning group
THEREFORE ...…read more

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An ultradian rhythm lasts for less than 24 hours and one example of this is sleep stages.…read more

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REM sleep is also known as the paradoxical sleep as the brain and eyes are active
but the body is paralysed
An issue with REM sleep is the assumption that it its dreaming sleep...
Dement and Kleitman = support of stages of sleep and
dreaming in REM
Woke participants up at times when brain waves were characteristics of REM
found that...
Participants were highly likely to report dreaming in this stage.…read more

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Infradian rhythm- Infradian rhythms last longer than 24 hours
Monthly cycle
The human female menstrual cycle is the most well known infradian rhythm
= regulates ovulation.
The menstrual cycle = involves fluctuating hormone levels of progesterone and
oestrogen. Progesterone causes an increase in the blood in the lining of the womb
and oestrogen allows for an egg to ripen and be released. The lining of the womb
thickens in preparation for a pregnancy.…read more

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Become depressed during the winter months and recover
during the summer
darkness =
melatonin = less
less serotonin
= depression
Understanding the role of darkness in SAD has led to effective therapies = such as phototherapy, using a
strong light in the evening or early morning to change levels of melatonin and serotonin.…read more

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= internally managed clocks which coordinate biological processes such as the circadian
sleep-wake cycle
The main endogenous pacemaker is the suprachiasmatic
Endogenous nucleus which lies on the hypothalamus
pacemakers interact
with exogenous The SCN obtains information about light from the
eye via the optic nerve
zeitgebers, such as
This occurs even when the eyes are shut, as light
light to entrain our penetrates the eyelids
If our endogenous clock is running slow, morning light
automatically shifts the clock ahead, coordinating with the…read more

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Research support
Morgan transplanted the SCN of `mutant' hamsters who had been trained to have 20
hour circadian rhythms, into norma hamsters who in turn adopted the same 20 hour
However there are 2 main issues with conducting animal research.
1.The first ethical issue is harm
2.The second is we cannot generalise the findings from animals to humans
Harm is an issue as a vital part of the animals brain is being damaged, meaning
that they cannot function adequately.…read more

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Shift lag
When people change to a different shift, their bodily processes are in a
state of desynchronisation. This is because shift workers have to
reverse their natural circadian rhythms and adjust their sleep-wake
cycle without the aid of zeitgebers such as light and social cues. The
fatigue associated with shift work and the disruption to sleep patterns is referred to as shift lag.
Decreased alertness
There is a higher rate of accidents at work at night due to decreased alertness.…read more


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