Human Circadian Rhythm


Circadian is a combination of two Latin words, “circa” that means about and “dia” that means day. The literal meaning of circadian is “about a day”. Simply put then, Circadian Rhythm is the regular flow of activity within a day. Human Circadian Rhythms are biological activities that follow a 24-hour cycle regulated by the human “biological clock” (Klein, et al 2007). This “biological clock” tells the body how to respond accurately to the changing settings in the environment. One of the most evident bodily functions related to the Circadian Rhythm is sleep. Sleep helps the body “re-set” at least every 24 hours. It is controlled by external cues, particularly the rising and setting of the sun. This is called the sleep-wake pattern. It relies on light (Silva, Albuquerque and Araujo 2005) to indicate time. The rising and setting of the sun is the most powerful control of the rhythms. This is the reason why humans are normally asleep at night and awake in the day.

            The Circadian Rhythm has both endogenous and exogenous components (Thomas 2003). The endogenous component is the human internal clock that is controlled by the brain’s hypothalamus. It sets cues as to when a certain body function is expected to run. On the other hand, the exogenous component comprises external cues from the environment. These external cues act as stimuli to body functions. Circadian Rhythms need both components to work. Regulated temperature is an example of the endogenous and exogenous components working together.

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            The onset of the Circadian Rhythms is believed to be during the fist 6 months of life (Pobojewski 2007). During this time the human body already adapts to certain cycles and set of normal activities. The body learns periods of sleeping, eating, excreting and maintaining of vital signs. Discrepancies in these cycles usually create an unwanted response. It is safe to assume that once the human body learns such cycles, it would be very difficult to break them. The Human Circadian Rhythm is as individual as humans themselves. No two Circadian Rhythm patterns are alike. Each is developed and enhanced according to the individual’s set of cues, both endogenous and exogenous. There are some instances when an individual has to ignore the Circadian Rhythm. This is possible only for a short period of time. Many diagnosed illnesses are often related to prolonged disruption of the Circadian Rhythm.

            Arguably the most common characteristic present among things with life, the Circadian Rhythm, is as prominent in animals as it is in human beings. The storing of nuts by a squirrel, the spinning of webs by a spider, the burying of eggs in the beach by a turtle, the feeling of hunger during lunch by a human being are all controlled by the Circadian Rhythms. In fact, animals rely heavily on their own “biological clocks” to complete many of their regular activities (Labak 2005).  Animals are driven by their “biological clocks” to mating so that they can give birth during the mild seasons, when food is abundant. The light/dark pattern is important to animals. Animals that rely on vision to forage for food are most active in the day. On the other hand, those that are capable of activity with minimal dependence to sight are usually active at night. Animals take their cues for migration and hibernation from the length of days that indicate the turn of seasons. What humans normally refer to as instincts in animals are actually Circadian Rhythms in action. They are the driving force to all animal activities.

            A number of studies have been conducted that correlates body functions with the Circadian Rhythm. Studies have been conducted to assess the effects of the rhythms to various mental and psychological disorders. It has also been scrutinized against many physiological illnesses, more specifically when the rhythms are disrupted. However in more recent times, the Circadian Rhythm has been studied for the purpose of performance enhancement. In fact, more and more studies are being conducted of the Circadian Rhythm and its relation to athletic performance. In many cases, the rhythms have been found to have a significant effect (Klein 2007). The Circadian Rhythm and its effects are now slowly being considered in design of training regimens and performance assessments.

            This paper aims to present a clearer understanding of the relationship between the Circadian Rhythm and human activity. Through experimentation, the significance of the Circadian Rhythm will be highlighted. In the end an inference of how to maximize this relationship will be drawn. So that performance, like in athletics, can be optimized.


Kline, C., Durstine, J., Davis, J., Moore, T., Devlin, T., Zielinski, M., and Youngsted, S.

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