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Lecture notes Biopsychology - Summary Circadian Rhythms
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Summary Circadian RhythmsSummary of Circadian Rhythms
One biological rhythm is the 24-hour circadian rhythm (often known as the ‘body clock’),
which is reset by levels of light. The word circadian is from the Latin ‘circa’ which means
‘about’, and ‘dian’, which means ‘day’.
The sleep-wake cycle is an example of a circadian rhythm, which dictates when humans and
animals should be asleep and awake. Light provides the primary input to this system, acting
as the external cue for sleeping or waking. Light is first detected by the eye, which then
sends messages concerning the level of brightness to the suprachiasmatic nuclei (SCN).
The SCN then uses this information to coordinate the activity of the entire circadian system.
Sleeping and wakefulness are not determined by the circadian rhythm alone, but also by
homoeostasis. When an individual has been awake for a long time, homeostasis tells the
body that there is a need for sleep because of energy consumption. This homeostatic drive
for sleep increases throughout the day, reaching its maximum in the late evening, when
most people fall asleep.
Body temperature is another circadian rhythm. Human body temperature is at its lowest in
the early hours of the morning (36oC at 4:30 am) and at its highest in the early evening
(38oC at 6 pm). Sleep typically occurs when the core temperature starts to drop, and the
body temperature starts to rise towards the end of a sleep cycle promoting feelings of
alertness first thing in the morning.
Evaluating Circadian Rhythms
Research Support: Research has been conducted to investigate circadian rhythms and the
effect of external cues like light on this system. Siffre (1975) found that the absence of
external cues significantly altered his circadian rhythm: When he returned from an
underground stay with no clocks or light, he believed the date to be a month earlier than it
was. This suggests that his 24-hour sleep-wake cycle was increased by the lack of external
cues, making him believe one day was longer than it was, and leading to his thinking that
fewer days had passed.
Individual Differences: However, it is important to note the differences between individuals
when it comes to circadian cycles. Duffy et al. (2001) found that ‘morning people’ prefer to
rise and go to bed early (about 6 am and 10 pm) whereas ‘evening people’ prefer to wake
and go to bed later (about 10 am and 1 am). This demonstrates that there may be innate
individual differences in circadian rhythms, which suggests that researchers should focus on
these differences during investigations.
Additionally, it has been suggested that temperature may be more important than light in
determining circadian rhythms. Buhr et al. (2010) found that fluctuations in temperature set
the timing of cells in the body and caused tissues and organs to become active or inactive.
Buhr claimed that information about light levels is transformed into neural messages that set
the body’s temperature. Body temperature fluctuates on a 24-hour circadian rhythm and
even small changes in it can send a powerful signal to our body clocks. This shows that
circadian rhythms are controlled and affected by several different factors, and suggests that
a more holistic approach to research might be preferable.
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