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

as winter or night-time, so that leaf opening and hibernation happen at exactly the right time. These external events are called zeitgebers. Behaviour has to respond to zeitgebers to adapt to what is happening in the natural world. Endogenous are sometimes called biological clocks. The main one in mammals (human beings), is the supra-chiasmatic nucleus (SCN). This is situated in the hypothalamus. The SCN has an inbuilt circadian firing pattern. This pattern continues even in cells of the SCN that have been removed from the brain and kept alive artificially. This proves that it is the SCN and not some other brain area generating them. The SCN causes sleep by stimulating the pineal gland to produce melatonin. Melatonin increases sleepiness by acting on the hypnogenic centres situated in various parts of the brain. Because the SCN generates a rhythm that lasts longer than 24 hours, it needs to be reset each day. The SCN is connected to the retina and it appears that daylight falling on the retina has the action of resetting the SCN in order to keep the sleep-wake cycle in line with night and day. 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. It indicates how we use stimuli around us to coordinate our biological clocks. Biological processes can also be influenced by other exogenous factors, such as, outside temperature and social patterns. For example, Eskimos have regular sleep/waking cycles, even though they have continuous daylight in summer and continuous darkness in winter. This shows that for them the social rhythms of life are the dominant zeitgebers. Most of the time zeitgebers, such as light or social behaviour patterns, change slowly if at all. There are times when they change radically and quickly. The usual coordination between our biological clocks and rhythms breaks down. Two examples are jet lag and shift work. If you went from England to the USA leaving at noon, you arrive at about 7pm UK time, but it would be 2pm USA time. All you psychological rhythms are working to UK time, so that at 6pm USA time you are ready to sleep, you would have a falling body temperature and decreasing bodily arousal. 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