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Can Your Body Temperature Tell the Time of Day? Find Out with Human Circadian Cycles.

Time Required Long (2-4 weeks)
Prerequisites None
Material Availability Readily available
Cost Low ($20 - $50)
Safety No issues


Have you ever had to adjust to a new time zone and noticed that it takes a while before you start to feel normal again? By shifting your sleep and activity schedule, you have altered the pattern of your body’s circadian rhythms. Human beings, like many other living things, have a number of internal processes that show a distinct circadian rhythm. The most obvious is our sleep cycle, with activity during the day, followed by sleep during the night. Circadian rhythms have also been demonstrated in humans for changes in body temperature, heart rate, alertness, and physical performance in sports. In this human biology science fair project, you will investigate human circadian rhythms by measuring body temperature and reaction time at different times of day.


In this human biology science fair project, you will measure how body temperature and reaction time vary throughout the course of a day.


David Whyte, PhD, Science Buddies

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Last edit date: 2012-12-07


In order to stay healthy and to function efficiently, living things must coordinate their internal processes with the external world. The most obvious feature of our environment that most creatures have to respond to is the daily cycle of light and dark. Biological processes that follow this 24-hour cycle, such as our sleep-wake cycle, are said to follow a circadian rhythm. One of the best-known circadian rhythms in humans is the daily change in body temperature. We tend to be at our coolest in the early morning and at our warmest in the late afternoon and early evening. Other circadian rhythms include hormone levels, alertness, muscle strength, and heart rate. The controlling regulator for these cyclic processes within the body is thought to be the hypothalamus, which is in the brain.

Overview of biological circadian clock in humans.
Figure 1. Overview of circadian cycles in humans. This diagram depicts some of the circadian patterns that occur in humans. Note that the clock is on a 24-hour cycle, so that 3:30 p.m., for example, is noted as 15:30. The hormone melatonin, which is a key regulator of the internal circadian clock, peaks in concentration at 21:00 (9:00 p.m.). (Wikipedia, 2008.)

Any change in the circadian cycle (such as jet lag and other conditions associated with travel) requires a certain period for readjustment. Jet lag can cause excessive sleepiness and a lack of daytime alertness in people who travel across time zones. Another factor that can affect circadian rhythm is work schedule. For instance, people who work at night are more prone to on-the-job injuries because their circadian rhythm is off-balance with the light and dark times of day.

According to William Garret (see Bibliography), "Reaction time peaks in the early evening at the same time as the maximum body temperature. This is partially because the nerve conduction velocity increases 2.4 meters per second (m/s) for every 1°C increase in body temperature."

Do you think Garret is correct that your reaction time is fastest in the early evening? When do you think it lowest? And how much difference is there? Find out in this human biology science fair project by measuring body temperature and reaction time at various times during the day. Reaction time will be measured using online tests that are based on how fast you respond with your mouse to visual cues on the computer screen. You will then chart your results to graphically show how your temperature and reaction times vary during the course of the day.

Terms and Concepts

  • Circadian rhythm
  • Core body temperature
  • Hypothalamus
  • Nerve conduction velocity
  • Reaction time


  • What physiological processes have circadian rhythms in humans?
  • Can you think of professions where a slight change in reaction time at different times of the day could be critical?
  • Research how the time of day affects performance in sports.


  • Garret, William E. Exercise and Sport Science. New York: Lippincott Williams & Wilkins, 1999, p. 354.
  • Chudler, E, University of Washington. (2008) Biological Rhythms—Experiment 1: The ups and downs of body temperature. Retrieved August 1, 2008, from http://faculty.washington.edu/chudler/clock.html

To learn more about the biology of circadian rhythms, visit this website at the University of Utah:

This test from the British Broadcasting Corporation (the BBC) tests your reaction time by measuring how fast you are able to shoot escaping sheep with a tranquilizer:

This website from the Exploratorium Museum, in San Francisco, California, tests your ability to react to a 90-mile-per-hour fastball:

Materials and Equipment

  • Thermometer to measure body temperature (digital is best)
  • A computer with Internet access (so you can use websites that have tests for reaction time)
  • Lab notebook
  • Graph paper

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Experimental Procedure

In these experiments, you will need to make sure you get the best possible data so that the variation in temperature and reaction time can be measured accurately. Your body temperature may vary from 1 to 2 degrees Celsius. Perform each trial three times and average the results.

  1. Set up a schedule for the times you will measure your body temperature and reaction time. Obviously you will need to work this into your normal schedule for sleeping and school. Be sure to get data early in the morning, say 5:00 a.m., since this is a critical point in your body's temperature cycle.
  2. Taking a reading every three hours is probably sufficient. You might make your measurements at these times: 5 a.m., 8 a.m., 11 a.m., 2 p.m., 5 p.m., and 8 p.m.
  3. Design a data table in your lab notebook to record your observations. It should include the following information: date, time of day, reaction time (three trials), body temperature (three readings), average reaction time, and average body temperature. Keep notes on your methods and observations, such as precisely how you measured your reaction time and body temperature.
  4. Try to control any variables that might affect your readings and give you poor results. For example, avoid drinking hot or cold beverages before you take your temperature.
  5. Lighting and noise levels should be the same or similar during all reaction tests.
  6. Try various thermometers to see which works best. The thermometer should be able to read the temperature at 0.1-degree intervals (or better). It is vital that you take your temperature accurately to be able to document the daily variation. Take your temperature three times each time and calculate the average for each time segment.
  7. To measure your reaction time, use a website with an online test. Here are two examples:
    1. This test from the British Broadcasting Corporation (the BBC) tests your reaction time by measuring how fast you are able to shoot escaping sheep with a tranquilizer: Sheep Tranquilizer Game.
    2. This website from the Exploratorium Museum, in San Francisco, California, tests your ability to react to a 90-mile-per-hour fastball: Hit the Baseball Test.
  8. Record the individual times and the average time for the reaction time trials. Set up the trials any way you like, but once you have a procedure you like, use it consistently. For example, each time you measure reaction time, do one practice test, then record the next three trials for your data.
  9. Repeat steps 2-8 on at least three different days.
  10. Graph your average temperature and reaction times.
  11. Did you observe a change in body temperature or reaction time during the course of each day? If so, how big was the change?
  12. Write a report on your observations. Some things you might include are: How accurately could you measure your temperature? What test of reaction time did you find worked the best? At what time of day was your body temperature highest? At what time of day was your body temperature lowest? Did you see a correlation between body temperature and reaction time?

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  • Test your friends and family, too (you could send your data table and instructions to them if they don't live nearby). Do you see any differences in circadian rhythms, based on age or gender?
  • Make more frequent observations, say every hour. Do you observe a small decrease in body temperature in the middle of the afternoon? Some people claim this dip in body temperature can explain cultural behaviors, such as afternoon siestas in Latin countries and the drinking of tea in the United Kingdom.
  • Research other types of physiological processes that are governed by circadian rhythms and devise ways to test them. Combine this data with the body temperature and reaction time data.
  • If you are not able to observe a circadian cycle in a physiological process, can you estimate how large the change would have to be for you to observe it (Hint: Look at the standard deviation of your data)? Include the standard deviation data as error bars in your graph.

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