Biological Rhythms

There are daily rhythms to many of our physiological functions and activities....our sleep, body temperature, alertness, neurotransmitter levels, etc. Many of these rhythms run on a cycle of about 24 hours. Rhythms that run on this 24 cycle are called "Circadian Rhythms". "Circa" comes from the Latin word meaning "around" and "dies" comes from the word "a day". Even if you lived in a dark cave and didn't know what time it was, the cycles would still exist and they would hold to a cycle of about 24 hours. Although neuroscientists are not exactly sure how the brain keeps track of time, there are several areas of the brain that may be involved. One of the possible "pacemakers" in the brain is a part of the hypothalamus called the suprachiasmatic nucleus.


Experiment 1: The Ups and Downs of Body Temperature

Grades 3-12

One circadian rhythm that is easy to keep track of is your own body temperature. Get an oral thermometer such as the one you use when you are sick. Make sure you know how to use it properly!! Measure your temperature every 2 hours from the time you get up in the morning to the time you go to sleep. (If you can't measure your temperature every 2 hours, then just measure it as often as it is feasible). Don't eat or drink anything right before you take your temperature. Make sure to take your temperature the same way every time and that you read the temperature VERY ACCURATELY....the differences in your body temperature are only a few 0.1 of a degree. Chart your body temperature with time...use the X axis for "Time of Day" and Y axis for "Body Temperature". Do you see a pattern?

An additional activity to do with the body temperature measurements is to combine the reaction time experiment. Measure your reaction time to catching the ruler and plot your "catch times" with your body temperature. Is there a correlation? Is your reaction time faster or slower with warmer body temperatures?


Did you know?

Normal body temperature can vary up to 2oF in a 24 hour period. According to the Guinness Book of World Records (New York: Bantam books, 2000, p. 263), the person with the highest body temperature who lived to tell about it is Willie Jones. On July 10, 1980, Mr. Jones was admitted to the hospital with heatstroke. His temperature was 115.7oF (46.5oC). After 24 days in the hospital, he was discharged. Body temperatures of 109oF can be fatal. (Statistic from Prevention's Giant Book of Health Facts, 1991.)


Experiment 2: Rhythms All Around

Grades 3-12

People have biological rhythms...what about other creatures? Many schools have classroom pets - a rat, rabbit, hamster, fish or frog. If your class has a pet, study its behavior to see if you can determine any cycling patterns. For example, if you have a rat, observe the amount of time it spends eating, walking and sleeping at different times of the day. Check on it every 2 hours and watch it for 10 minute periods. It is best if a group of students helps out. Assign 1 behavior to each student. Therefore, one person can measure the amount of time spent eating, another person can watch for sleeping, etc. Do not disturb the animal while you are observing it. Chart the amount of time spent in each behavior at different times of the day. Keep track of it for several days. Are there any consistent patterns?



Experiment 3: Built in Stopwatch

Grades 3-12

Sometimes it seems as if time flies by....sometimes it drags on forever. How good are you at estimating time? Do you have a built in stopwatch? Here are two ways to test the built in stopwatch:

  1. Estimate the length of a period of time, for example, one minute. Tell your subject to count to 60 by ones at a rate of one per second. When you say "Go," start your timer. When your subject gets to 60, stop the timer. How close to 60 seconds was your subject?

  2. Choose a period of time, but don't tell your subject how much. Do tell the person that you will start timer and then stop it after a period of time. Say "Go" and start your timer. Your subject should begin counting at a rate of one per second. After a period of time, stop the timer and say "Stop." Ask your subject how much time has passed.

Try several different time periods (5 seconds, 15 seconds, 30 seconds, 60 seconds, 90 seconds) and see if performance gets better or worse.


On-line Time Estimation Experiment - How accurate are you at estimating time?


Experiment 4: Built-in Alarm Clock

Grades 3-12

How many times have you set your alarm clock for a particular time only to wake up a few minutes before it rings. You find out you didn't need the alarm clock at all because you woke up without it. How good is your "built-in alarm clock"?

This experiment is best done in the summer on a weekend or some other time you don't have to wake up at a special time. Before you go to sleep, determine what time you want to wake up, but do not set your regular alarm clock. The wake up time should be a time that you usually wake up. For example, if you wake up to go to school at 7 am., tell yourself to "Wake up at 7 am." Then just go to sleep and see what time you wake up.

If you have a few days to try this, keep track of your wake up times to see if you come closer to your built-in alarm clock's setting. You could even graph how far off you were from the time you wanted to wake up by graphing number of minutes away from your built-in alarm clock's setting. For example, if on the first day you woke up at 7:10 instead of 7:00, you would be off by 10 min. If on the second day you woke up at 7:08 instead of 7:00, you would be off by 8 min. See if there are any patterns to your graph.


Don't blame me if your built-in alarm clock doesn't work to wake you up. DO NOT do this experiment when you have to wake up a special time (such as a school day)! Your built-in alarm clock may FAIL and you will be LATE. You may want to set a real alarm clock as a back up system!


Experiment 5: A Yawner

Grades 4-12

No one really knows the cause and purpose of yawning. However, yawns do appear to follow a daily cycle. Yawns seem to occur most often soon after waking and about an hour before bedtime. See if your yawning behavior has a similar cycle.

Keep track of the number of your yawns and the time that they occur from the time you wake up to the time you go to sleep. Print out this YAWN worksheet to record your yawns. The most difficult part of this experiment is remembering to record every one of your yawns, so keep the worksheet is a handy place.

Add up the number of yawns that occurred in each half hour period and plot them out on a graph. Use the time of day on the X-axis and the number of yawns on the y-axis. Compare your graph with the graphs of other people.


See Circadian Technologies for more about nature's rhythms.

BACK TO: The Senses Experiments and Activities Table of Contents

Send E-mail

Get Newsletter

Search Pages

Donate to
Neuroscience for Kids