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ESSAY Discuss the role of circadian rhythms in human behaviour
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Discuss the role of circadian rhythms in human behaviour (16 marks)A circadian rhythm is a type of biological rhythm subject to a 24-hour cycle, for example the
sleep/wake cycle. The dominant exogenous zeitgeber – an external influence that entrains our biological
rhythms – in the sleep/wake cycle is light. Light is received by the photoreceptor cells of the optic
chiasm, which is located just below the suprachiasmatic nucleus (SCN): the main endogenous pacemaker
in the cycle. This causes the SCN to reset in the presence of light, ensuring that synchrony between the
body and the environment is maintained by the mechanism of photo-entrainment. Also, the SCN
stimulates the pineal gland to secrete melatonin, a hormone that induces sleep. Melatonin production is
inhibited by light, explaining why we feel more wakeful during daylight hours.
Another circadian rhythm is core body temperature, which peaks at around 6pm, and is at its
lowest at around 4am. This rhythm affects human behaviour, as a higher core body temperature is
associated with better cognitive performance. This is demonstrated by Gupta’s research, which showed
that performance on an IQ test was better at 7pm as opposed to 9am and 2pm.
The role of circadian rhythms in human behaviour is strengthened by Siffre’s research. After
living in a cave with no natural light for two months, Siffre’s sleep/wake cycle settled to 25 hours. This
supports the idea that our internal body clocks are strong and have a regular pattern, but need to be
entrained by exogenous zeitgebers to maintain synchrony with the environment. However, as this study
only involved one participant, it may not be appropriate to generalise the findings universally; for
example, Siffre repeated his study when he was 60 years old and found that his sleep/wake cycle settled
at 48 hours, demonstrating the effect of age on circadian rhythms.
Our understanding of circadian rhythms is limited, as we may be putting too much emphasis on
the influence of light. Luce and Segal suggest that social cues are a more important influence, as
individuals who live in the Arctic experience white nights and days with no light at all, yet still have
regular sleep/wake cycles. However, Miles recounts a study of a man who was blind from birth, who had
a 24.9 hour sleep/wake cycle despite being exposed to social cues, reinforcing the idea that light is the
dominant zeitgeber.
Our knowledge of circadian rhythms, and the consequences of their disruption, is strengthened
by its practical applications to shift workers. Czeisler studied a chemical plant in Utah, where there were
high levels of stress, health and sleep problems. By changing shift patterns so that they moved forwards
every 21 days, rather than backwards every 7 days, employee output and satisfaction was increased
after nine months. Our knowledge of circadian rhythms is therefore valuable as we can increase worker
productivity, health and satisfaction.
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