Describe and Evaluate Circadian Rhythms

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Describe circadian rhythms (8marks)
Circadian rhythms occur every 24 hours; an example of a circadian rhythm is the sleep-waking cycle. We are diurnal animals who are active during the daytime and asleep at night, other animals are nocturnal they are active at night but asleep during the day. The circadian rhythm depends on the interaction of physiological and psychological processes to be tuned into the sleep-waking cycle so energy is provided when needed. As diurnal humans we have a fairly stable sleep pattern with the time we go to sleep and the time we wake up, this consistency suggests an internal mechanism controls sleep, endogenous pacemaker. However, this can be overridden by external factors, exogenous zeitgebers. The endogenous pacemaker that works as an internal body clock to regulate biological rhythms of the sleep-waking cycle is the suprachiasmatic nucleus (SCN, a small group of cells in the hypothalamus). The SCN has a direct pathway to the retina of the eye, in which light enters. Light is an exogenous zeitgeber which also has an influence on the rhythm due to being synchronised with the SCN. The SCN is also connected to the pineal gland by a neural pathway; the pineal gland is where the hormone melatonin is manufactured. Melatonin affects sleep patterns, the more melatonin released due to low levels of light the more sleep, when melatonin levels drop due to high levels of light the individual wakes. However, sleep/wake cycles can be maintained in the absence of light as the SCN is free-running, it follows its own innate rhythm and continues to follow a pattern of circadian activity even when isolated from the rest of the brain.

Discuss research into circadian rhythms (16marks)
One of the earliest scientific studies on endogenous pacemakers was conducted by Stephan and Zucker (1972). They wanted to investigate the effects of damage to the SCN (of rats) on circadian rhythms; they deliberately damaged the SCN via surgical controlled lesions....
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