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Circadian are the rhythms which vary according to the time of day. Through extensive research it has been proved that our physiological functions normally fluctuate in cycles commonly referred to as rhythms. These rhythms are regulated by an internal biological clock located in the human brain. The time range of the rhythms differs depending on the process under consideration. This can be in minutes considering the stages of sleep and in days while considering the female menstruation cycle. The term circadian is a combination of two Latin words circa meaning around and diem meaning day. The study of the biological temporal rhythms is referred to as chronobiology.


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The circadian rhythms vary according to the time of the day and include phenomena such as body temperature, blood pressure and urine production in humans. Plants also exhibit the circadian rhythms with a good example being opening and closing of flowers. In human beings researchers have proved that the main physiological functions which include core body temperature, heart rate, blood activity, hormone production, sleep/awake cycle and the gastric activity all have rhythms of approximately 25 hours which are the circadian rhythms. This is because they are occurrences that occur after a certain period of time repetitively and thus the name cycles. This activity has been investigated in caves and modern sleep laboratories where the environment is absolutely time free to ensure that the results are not biased due to the subject under investigation responding to the time changes. The results obtained from these studies show that the human body is governed by some built-in time setters or clocks which is referred to as an internal body clock. It is seen that more than 100 body functions operate between their maximum and minimum values in a day. They fluctuate between these two points and this result in the human functioning taking about 25 hrs (approximately one day) to complete. This occurrence made Dr. Franz Halberg in 1959 to use the Latin word circadian which in essence means “approximately one day” in describing these changing body functions.

The easiest measured circadian rhythm is the human body temperature. Generally humans who are healthy experience rhythmic variations in their body temperature and this happens in each and every day. A huge percentage of people have a difference of about two degrees Fahrenheit which ranges between 97o and 99o. The lowest values normally occur during the morning hours of the night about 2:00am to 5:00am and the highest values being recorded during the evening hours about 7:00pm to 10:00pm. This has been investigated and results show that this variation occurs in the same pattern after every 25 hours which is approximately one day and this is independent of the time of sleep and the time of waking up. In a normal condition, these circadian rhythms are liked or synchronised together by the internal biological clock and are entrained to an external clock time by the external time cues which are the synchronizing effects to the physical world which we live in. These are the variation in the sunlight levels and the increased activity around us caused by environmental factors and family. The external time cues are known as zeitgebers which is a German word and they can either be physical or social in nature with the most powerful being the changing light levels in the environment. Any disruption in this natural time pattern can cause the zeitgebers to act upon the circadian system to restore it to the normal state and in synchrony with the time patterns or discourage the adjustment to the newly acquired routine. This is why it is so easy to stay awake during the holidays but after the holiday is over it is difficult to resume the normal schedule of waking up early. The different circadian rhythms change at various different rates and this is achieved with greater difficulty. This is what normally causes the jet-lagged feeling which comes about when one tries to change to a nocturnal schedule and vice versa. It can be compared to an orchestra which starts with one conductor playing the music in harmony but results in a cacophony when one conductor enters the music with a different beat since the music now converts to different rates. This  creates a jet-lagged feeling when the different rhythms are adjusting to the different rates and this is inevitable. This is similar when one tries to adjust ones body to a nocturnal schedule which creates the body clocks to lose their synchronisation causing circadian disharmony.  This results since the body is trying to change the way in which it was used to operate during daytime when it was most active to the night shifts which now become the most active shifts where the body should be alert.

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