What Is Biological Clock


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.

The onset of the Circadian Rhythms is believed to be during the fist 6 months of life (Pobojewski, 2007).

Get quality help now

Proficient in: Medicine

5 (339)

“ KarrieWrites did such a phenomenal job on this assignment! He completed it prior to its deadline and was thorough and informative. ”

+84 relevant experts are online
Hire writer

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.

Materials and Method Section

Animal physiology 433 laboratory class designed a lab experiment to study biological rhythms. The study of biological rhythms took place during the winter quarter of 2008. There were eighty-nine students selected to participate in this study. Four types of measurement tests were chosen to study rhythms. These tests included measurements of pulse rate, eye-hand coordination, adding speed, and body temperature.

The measurement were collected every two hours during the 24-hour period. Each student had one week to gather their data. While collecting the data, the student was required to disturb their “normal sleep and awakening time”, meaning that students had to wake up in the middle of the night or interrupt their daily schedule to collect data.

The collection data during the rest phase needs to be gathered right away when student wakes up without getting up from the bed. It is recommended that each student divided their rest phase measurement for different nights, instead of taking all measurements in one night.

They performed the pulse measurement. There were several ways to measure the pulse rate. One way was to place the index finger on the side of the wrist, just above the wrist bone, and press lightly. The beats were counted for one minute or for 10 seconds and multiplied by six. Another way to measure pulse was by taking the pulse from the artery to the right or left near the Adam’s apple and counting the beats per minute.

Students performed the eye-hand coordination test. Using the dominant had, touch the index finger with the thumb and count “1”, then take the thumb and touch the middle finger and count “2”. Once the thumb is touching the little finger get to the little finger, count “4”. Now count by going backward, first touching the ring finger and count “5”, then the middle finger and count “6” and to the index finger and count “7”. Perform these procedures until have reached the count to “25”. At “25”, the thumb and the index finger must be touching each other. Try to practice the movement before doing the actual measurement. If a student makes a mistake then is required to start counting over again. Student needs to record how many times it takes to complete the correct count.

Student measured adding speed. For this test, the required material is a table of one digit numbers placed in columns. Using a piece of paper, the student should place it at the bottom of the second row of numbers. As the student moves the paper down to the third row the student needs to add the first and the second row numbers. Move the paper down to the third row and add second and third line. Add only two lines at the same time by continuing to count until added the entire column. Finally record the time that is took to complete this procedure. Use different column for each circadian time period.

Measurement is oral temperature. Student should beging by using a reusable thermometer to measure the temperature. Taking the thermometer and place it under the tongue for one minute. After a minute passes readily read the thermometer by looking at the last black dot, it will show the temperature in Fahrenheit. Finally, rinse the thermometer with soapy water or wipe with rubbing alcohol. Dry it and store in the thermometer case.


The study of circadian rhythms over the 24-hour period has shown the following results. According to Figure 1, the highest heart rate was recorded between 1200-2000 and lowest heart rate was at 400. The maximum heart rate was approximately 78 beats per minute during the hours 1200-2000 or from 1000-2200. The minimum heart rate was at 0400 about 65 beats per minute or between 0200-0600.  The mean heart rate is significantly different between midnight and daytime (P<0.05, Turkey’s HSD test).

The highest eye-hand coordination count is at 400 in the morning and lowest at 1200. It takes about 11 sec to perform eye-hand coordination test at 0400 or between hours of 0200-0600. And the minimum is at 1200 (7 sec) or between hours of 0800-2200.The mean eye-hand coordination is significantly differ at 0400 than eye-hand coordination count between noon (Figure 2, P<0.05, Turkey’s HSD test).

The maximum number of mistakes is at 0400 (>1 number of mistakes) or between 0200-0400. On the other hand, the minimum is from 0600-2400 (between 0.2-0.6 number of mistakes). The mean number of trials with mistakes have differ significantly during the night that during the daytime (Figure 3, P<0.05, Turkey’s HSD test).

The highest values for adding speed occur at 0400 and lowest value is between  1200-1400 and 2000. Being maximum at 0400 (77 sec) or from 2400-0800 and minimum is between 1200-1400 and 2000 (approximately 53 sec) or between 0600-2400 The mean adding speed is significantly differ at 400 than during the daytime (Figure 4, P<0.05, Turkey’s HSD test).

The highest body temperature observed between 1400-1800 hours and the lowest at 0400. Maximum at 1400-1800 (97°F) or 1000-2200 and minimum at 0400 (96°F ) or 2400-0800. The mean oral temperature at midnight is also significantly lower than temperature between 1400-1800 (Figure 5, P<0.05, Turkey’s HSD test).


In the past, several studies have been conducted to demonstrate the diurnal variations that occur in the heart beat.  One of the main factors that control the heart beat include the two components of the autonomic nervous system, namely, the sympathetic component and the parasympathetic component.  Both these components have opposing actions.

A study conducted by Stein et al (2006) demonstrated that the heart rate variations are very much dependent on the circadian cycle compared to the ultradian cycle.  Stein et al involved about 113 subjects in his study between the ages 48 to 68.  He included about 68 men and 48 women.  One of the main instruments utilized in his study was an overnight polysomnographic ECG, which could help determine the heart rate and record the diurnal variation.

In another study conducted by Bonnemeier et al (2003), demonstrated that during vagal stimulation had a significant role over the heart rate.  In turn, the vagus stimulation was controlled by the circadian cycle.  With age, there was a decrease in the vagal stimulation leading to decreased in the parasympathetic activity at night times.  Bonnemeier et al utilized 166 healthy subjects who did not suffer from any cardiac disorder.  About 85 women and 80 men, between the ages of 30 to 55 years formed a part of this study.

Several external and internal factors helped to control temperature including circadian factors, menstrual factors, etc.  Sund-Levander et al who conducted several experiments on temperature variation found that the time of the day and then season, had a significant role over temperature.  Recently, it was found that time of the day controlled the temperature from an internal mechanism rather than an external.  Between the 1960’s, 1970’s and the 1980’s, more than 2700 articles have been published the internal role the circadian cycle has over body temperature variation.  Some of the factors that had a role over the circadian cycle included the light and day cycles and the meal timings.  Earlier study conducted by Kelly Greg et al in 2006 demonstrated that the temperature of the body was lowest between 3 am to 6 am and highest between 4 pm to 9 pm.  The difference between these two temperatures was about 1.8 degrees F.  Greg also found that several other factors could affect this variation in temperature.

Although no current evidence is available to demonstrate that maximum number of human errors occurs during 2 am to 4 am, current literature suggest that the human tendency to make mistakes is higher during the early morning period.  This is because relatively fewer research studies have been conducted in this area.  One of the vital areas in which attention is required by the personnel is the night shifts in the trauma unit of a hospital.  Studies have shown that as these personnel experience a disruption of the circadian cycle, they are bound to make mistakes in the early morning hours.  Fatigue has been one of the causes for these errors.  In the past, 4 major disasters in the World have occurred due to disasters by the personnel during the early morning hours.  Scientists suggest that the human body circadian cycle is tuned in such a way that during the early morning hours, human physical and mental activity is minimized (Duncan, Jha, & Bates, 2001).

A study was conducted by Edwards et al (2008) to determine the level of accuracy at different times of the day in order to determine the effect on circadian cycle.  The individuals had to aim at a target and reach the center of the target as close as possible.  This task required a great level of eye-hand coordination.  Before the performance of the task, the oral temperature was recorded.  This study demonstrated a close relationship between the oral temperature and performance levels (Edwards et al, 2008).

Thus it can be said that the findings of this study have been consistent with that of the past studies.  The results of this study seem to be very specific and precise, as it points out to specific periods in a 24-hour cycle.  Such data needs to be studied further through advanced studies, and of in consistent results are obtained, could the results of this study be utilized for practical purposes.  Besides, long-term studies need to be performed in individuals with a disturbance in the circadian cycle.


  • Bonnemeier H, Richardt G, Potratz J et al (2003). ”Circadian profile of cardiac autonomic nervous modulation in healthy subjects: differing effects of aging and gender on heart rate variability.” J Cardiovasc Electrophysiol, 14(8), 791-799. http://www.ncbi.nlm.nih.gov/pubmed/12890036?ordinalpos=1&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVAbstractPlusDrugs1
  • Duncan, B. W., Jha, A.K., & Bates, D. W. (2001). “Chapter 46. Fatigue, Sleepiness, and Medical Errors.” In. Shojania, K. G., Duncan, B. W., McDonald, K. M. et al (Ed). Making Health Care Safer – A Critical Analysis of Patient Safety Practices, Rockville: AHRQ. http://www.ahrq.gov/Clinic/ptsafety/index.html#toc
  • Edwards, B., Waterhouse, J., & Reilly, T. (2008). “Circadian rhythms and their association with body temperature and time awake when performing a simple task with the dominant and non-dominant hand.” Chronobiol Int., 25(1), 115-32. http://www.ncbi.nlm.nih.gov/pubmed/18293153?ordinalpos=1&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVDocSum
  • Kelly, G. (2006). “Body Temperature Variability (Part 1): A Review of the History of Body Temperature and its Variability Due to Site Selection, Biological Rhythms, Fitness, and Aging.” Alternate Medicine Review, 11(4). 278-293. http://www.thorne.com/altmedrev/.fulltext/11/4/278.pdf
  • Stein, P. K., Domitrovich, P. P., Lundequam, E. J. et al (2006). “Circadian and ultradian rhythms in heart rate variability.” Biomed Tech(Berl),51(4),155-158.

Cite this page

What Is Biological Clock. (2018, Sep 16). Retrieved from https://paperap.com/paper-on-circadian-rhythm-essay/

What Is Biological Clock
Let’s chat?  We're online 24/7