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Discuss the role of the endogenous and exogenous factors in biological rhythms.

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Discuss the role of the endogenous and exogenous factors in biological rhythms. Patterns of behavioural and physiological activity in most organisms vary in a cyclical way. A circadian rhythm is a bodily rhythm that lasts about 24 hours. Other bodily rhythms can be infradian (lasting more than 24 hours) or ultradian (lasting less than 24 hours). A lot of examples suggest that these rhythms are inbuilt (endogenous). For example, in countries with bad winters, the squirrel population hibernates. If these squirrels are kept in a laboratory, in a constant warm environment, with altering 12 hour periods of light and dark, they go through the hibernation routine at the appropriate time of the year, increasing food intake and body weight and decreasing body temperature. In addition, they awake as spring approaches in the world outside. However, even with these endogenous pacemakers, in the real world behaviour has to be adapted to external events exogenous factors) such as winter or night-time, so that leaf opening and hibernation happen at exactly the right time. ...read more.


A study was with Michael Siffre to see the effects of removing light as a zeitgeber and allowing the biological clock to run free. In 1972 he spent six months in an underground cave in Texas separated from natural light / dark cycles. He was wired up so that various body functions could be recorded. When he was awake, the experimenters put his lights on; when he went to bed, they turned the lights off. He ate and slept whenever he wanted. At first his sleep-waking cycle was very erractic, but it settled down to a fairly regular pattern with a periodicity of between 25 and 30 hours. When he emerged it was the 179th day, but by his days, it was only the 151st! Studies like this show that subjects with free-running biological clocks settle to a rhythmic sleeping/waking pattern of about 25 hours, slightly longer than under normal conditions. This shows that our endogenous mechanism can control our sleep/waking cycles in the absence of light and that the presence of light as a zeitgeber is necessary to re-set the clock everyday so that the biological rhythm is perfectly coordinated with the external world. ...read more.


This produces jet lag. Studies show that the quickest way to recover from jet lag is to immediately adopt the local zeitgebers. Jet lag is more severe when travelling West to East. This may because it is easier to adjust our body clocks when they are ahead of local time. Because of melatonins role in controlling bodily rhythms, it has been studied as a possible treatment for jet lag. There is some evidence that it may help. (Kalat 1998) . The problems of jet lag and shift work are due to the way we have artificially dislocated the normal coordination between our biological clocks and the external world. Seasonal changes also effect people, with some developing Seasonal affective disorder. (SAD - clinical depression over the winter.) The key changes which are seen to trigger this are day length or temperature. These could interact with the zeitgeber to produce the depressive state. One therapy that has been found effective is is light treatment, involving exposure to bright white light for at least an hour everyday ...read more.

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