Skip Navigation

Living Clocks

Living Clocks
  • Grades:
  • Length: Variable


Students learn that some behaviors and functions of living organisms vary predictably every 24 hours, and that many life functions are governed by internal "clocks."

This activity is from The Science of Sleep and Daily Rhythms Teacher's Guide, and was designed for students in grades 6–8. Lessons from the guide may be used with other grade levels as deemed appropriate.

Teacher Background

Most living things behave predictably in cycles of about 24 hours, the period required for Earth to complete one full rotation. These cycles are called circadian, from the Latin words for “about” (circa) and “day” (dies). In this activity, students will explore circadian patterns in humans, animals and plants.

There are many familiar circadian rhythms in nature. Well-known examples include the flowering of morning glories at dawn and the nighttime hunting routines of owls. These behaviors are governed by internal mechanisms, often called “biological clocks,” within the cells of living organisms. Biological clocks that run on a 24-hour cycle also are known as “circadian clocks.”

The circadian timing system is complex and operates throughout the body. In fact, circadian clocks are part of our genetic code, and they govern virtually all functions of the human body. Examples include alertness, waking and sleeping, body temperature (lower in the morning just after waking, and higher in the afternoon), physical performance and hand/eye coordination, secretion of some hormones, and urine production. These cycles occur regularly over intervals of approximately 24 hours. Without cues from the environment, the human circadian clock eventually drifts into a cycle that is slightly longer than 24 hours.

Objectives and Standards

Life Science

  • Regulation of an organism’s internal environment involves sensing the internal environment and changing physiological activities to keep conditions within the range required to survive.

  • Behavior is one kind of response an organism can make to an internal or environmental stimulus.

Earth and Space Science

  • Most objects in the solar system are in regular and predictable motion. Those motions explain such phenomena as the day, the year, phases of the moon and eclipses.

Science, Health and Math Skills

  • Measuring

  • Observing

  • Drawing conclusions

Materials and Setup

Materials required for each student group will vary, depending on the investigation(s) being conducted.

Body Temperature Investigation

  • Digital thermometer with several sterile covers (and access to a fever thermomete at home)

  • Copy of student sheet (see Lesson pdf)

Bean Leaf Investigation

  • Source of natural sunlight, or fluorescent “grow light” with timer

  • 4 bean plants per group (purchase or grow in small pots from seed)

Animal Behavior Investigation

  • Study animals that can be observed in the classroom throughout the day (gerbils, birds, crickets, etc.)

  • Science journal or graph paper

Alertness, Heart Rate (or other student selected investigation)

  • Stopwatch or timer (if necessary)

  • Science journal or graph paper

  • Other materials as needed


  1. Read "Procedure," prior to class discussion of investigation options.

  2. Conduct discussions as a class.

  3. Have students work in groups of 2–4.


  • Follow proper care guidelines for any animals in the classroom.

  • Always follow district and school laboratory safety procedures.

  • It is a good idea for students to wash their hands with soap and water before and after any science activity.

Procedure and Extensions

Time: Several 30-minute sessions, depending on the options selected.

  1. To prompt students’ thinking about daily rhythms in themselves and other organisms, ask, What are some animal behaviors that follow the same pattern every day? [Rooster crowing, birds singing in the morning, bats coming out at night, etc.] Are you more alert or sleepy at certain times each day? Have you observed flowers that are open only in the morning?

  2. Discuss as a class how behaviors of living organisms are tied to the 24-hour cycle of night and day on Earth.

  3. Tell students that they will be investigating different daily cycles in plants, animals or themselves. You may assign each group an investigation from the list of activities (given below), or allow groups to devise their own topics for investigation.

  4. Have the members of each group decide upon a central question that will guide their “research.” For example, students conducting the body temperature investigation might ask, "Is body temperature constant throughout the day?"

  5. Students should plan the times of day at which they will conduct their measurements, the instrument(s) needed (thermometer, stopwatch, “grow light,” etc.), and the recorded units of measurement (degrees F, seconds, etc.). A sample data sheet for the body temperature investigation is included at the end of this activity. Students will need to create their own data-recording tables for other investigations.

  6. Have students work in collaborative groups to conduct their investigations over several days.

  7. After students have concluded their investigations, have each group present its question and observations to the class. Students should be able to describe the behaviors or body functions observed, methods used to measure the behaviors or functions (recording leaf movement, checking temperature, observing active times, etc.), and the pattern or patterns they discerned.

  8. Discuss the results of the different investigations with the class. Ask questions to prompt students’ thinking: What did all of the cycles you observed have in common? Did any of the patterns occur without the presence of normal sunlight? Do you think something other than sunlight controls the patterns you observed? Help students to reach the conclusion that something inside each organism controls plant and animal behaviors. Point out that most organisms have internal “timers” that regulate many aspects of their lives, and that these “timers” have a genetic basis.

Alertness or Heart Rate Investigation

Many physical activities and abilities—among humans and non-human organisms—vary predictably by time of day. For example, students may choose to observe and chart whether they feel alert (wide-awake; fully aware and attentive) or drowsy (sleepy) on an hourly basis over the course of several days; measure and record their resting heart rate at different times of day; or observe their “brain power” by timing how long it takes to mentally add columns of 50 single-digit numbers at different times of day.

Animal Behavior Investigation

If you have a hamster or gerbil with an exercise wheel in your classroom, your students can observe and record the times of day when the rodent is active. Or, have students observe and record the daily behaviors of other classroom animals, such as fish, crickets (will chirp at approximately the same time each day) or birds. Students should record eating, resting and active times over several days to determine if the animals’ activities follow a predictable pattern. The best results will be obtained if student observations do not disturb the animal subjects, and if the animals are exposed to a consistent cycle of light and darkness each day.

What students will observe: Animals may show a variety of predictable behaviors. For example, most animals are active at certain times of the day and more inclined to rest at others; birds and crickets will sing or chirp at similar times each day; and most animals tend to feed at particular times of the day.

Bean Leaf Investigation

Grow young bean plants from seed, or purchase them from a greenhouse. Before using plants grown from seed, be sure they have at least two leaves in addition to the cotyledons (fleshy seed leaves). Place the plants in a sunny window or a growth chamber with a light timer. Have students note the orientation of the larger leaves as early as possible in the morning and again later in the afternoon. In particular, students should notice whether the leaves are extended outward or folded downward toward the stem. Have students repeat their observations over several days. Then, place the plants in a darkened corner of the room or cupboard, and have students observe and record the position of the leaves at the same times as before.

Note. Students may notice that their plants’ stems curve toward the source of light. This movement is governed by chemicals inside the plant that cause cells on the side away from the light to lengthen more than cells on the side facing the light. This phenomenon is different from daily leaf movements, because it produces a permanent change in the shape of the stems.

What students will observe: Leaves will be dropped toward the stem very early in the day and fully extended (horizontal) later in the day, whether the plants are exposed to the sun or artificial light. These patterns will continue when the plants are kept in the dark.

Body Temperature Investigation

Have students measure their body temperature at three different times per day and repeat the process over three days. Times should be selected in advance and temperatures taken at the same time each day. Suggested times: immediately after waking in the morning, eight hours after waking (about 2:00 p.m.), and just before going to bed in the evening. Measurements should be recorded as degrees F, and should be made no less than 15 minutes after students eat, drink or brush their teeth. Have each student calculate and graph the average temperature of his or her group for each time of day.

What students will observe: Body temperature can be as much as one to two degrees lower in the very early morning than in the mid to late afternoon. This pattern is relatively consistent across individuals. Depending on levels of activity during the day, and the particular activities undertaken, student results may vary. Results also will be affected by the specific times at which temperature is measured.


Indiana University’s “Plants-In-Motion Theater” website offers time-lapse video of circadian movements of plant leaves ( Have students compare the behaviors shown on the website with those observed in their own plants.

Related Content


National Space Biomedical Research Institute

National Space Biomedical Research Institute

This work was supported by National Space Biomedical Research Institute through NASA cooperative agreement NCC 9-58.