Tick-Tock: Biological Clock
The Internal Clock
and a Sense of Time


1 period each for first 3 activities; 1 week+ for each of last 2 activities

chart paper


One of the most important features of a migratory animal is its ability to "tell" time. As you will see during the migrations this spring, an animal's very survival depends on being at the right place at the right time. All living things, including humans, are guided by internal mechanisms known as biological clocks. These "clocks" signal when to wake, sleep, eat, bloom, hibernate, migrate, and more! While the science of biological clocks is complicated and not fully understood, we do know that the sunlight (especially the number of daylight hours, or photoperiod) is the most powerful synchronizer or these daily, monthly, and annual rhythms for plants and animals.

Animals also respond to external cues, such as temperature and availability of food, but internal clocks are hard-wired for survival. For instance, if a hibernating animal waited to put on fat until the weather got cold, it might not have time to store energy for winter. But its biological clock signals the brain to pack the food in at the right time.

Overview: Students explore the concept of time and their own internal abilities to keep time. Then they consider the importance of biological clocks to migratory species.

Laying the Groundwork
Ask, Who is able to tell time without looking at the clock? If anyone responds, ask, What clues do you use? Write their responses on a chart and invite the class to try their hands at being time detectives.

Use some of the following activities to spark students' thinking about daily and annual rhythms.

Activity 1: Telling Time From the Gut!

  1. Cover the classroom clock and ask students who wear watches to take them off for the day.
  2. Have students write the numbers 1-10 on a piece of paper. At random intervals during the day, ask them to guess the time and record their guess on the paper. In the space next to the time they guessed, have them list the internal clues (such as hunger, fatigue, restlessness) and external clues (school bells, movement in the hallway, smells from the lunchroom) that they used to judge the time.
    Adaptation for Younger Students: Rather than ask students to guess specific times based on internal clues, ask relevant questions at particular times. For example, right before lunchtime, ask, How are you feeling inside? Why do you think you feel like that?
  3. At the end of the day, see how accurately students were able to estimate the time. Have them compare the clues they used and add new information to the chart you started earlier. Ask, How easy was it to tell time without looking at a clock? Which internal or external clues were most useful? Discuss whether students who do or do not wear watches had an easier time and explore what that might be so.

Activity 2: Sunrise, Sunset

  1. On the same day, find out whether students know how daylight changes during the course of a year. Ask them to record the following dates on a piece of paper: February 1, April 1, June 1, August 1, October 1, December 1. For each date, they should record the approximate time they think the sun rises and sets.
  2. Using a calendar with sunrise and sunset data to provide the answers, have students compare these guesses to the actual times and amount of daylight on those days.
  3. Ask, How did the data compare with your predictions? What general statement can you make about changes in daylength (photoperiod)? Why do you think it might be important for a migratory species to react to changes in photoperiod? (See the Mystery Class project for a full study of photoperiod.) Do you use the sun to tell time? How?

Activity 3: Time Awareness
Ask all students to stand. Divide the class into two groups according to the following characteristics: Those who usually wear a watch move to one side of the room. Those who don't wear watches move to the other side. Then divide each in half again using this criteria: students who wake up with an alarm and students who wake up naturally. If these four groups are too large for a discussion, break into triads or foursomes. If they're too small, use just two groups. Have each group discuss these types of questions:

  • What interesting experiences have you had because of oversleeping, jet lag, or changing to daylight savings time?
  • Do you usually wake up at about the same time every day?
  • Do you have pets that seem to be able to tell time? How do they behave? What clues do you think they use?
  • Does time seem to pass more slowly at the dentist's office or when eating dessert? What does this mean?
  • How would life be different without clocks?
  • Is there any correlation between your performance on the time tests in Activity 1 and the groups you are in?

Activity 4: Sleep Research
Try this sleep experiment with students.

  1. During the week, students record the number of hours they sleep each night.
  2. On Friday each calculates the average for the week.
  3. On the weekend, each student records the time he or she goes to sleep on Friday night and the exact time of waking up on Saturday morning. (Ask students to wake up on their own; they shouldn't use an alarm or be woken by another person.)
  4. Discuss how the number of hours each student slept on Friday night compares to the average number of hours he or she slept during the past week. Ask, What patterns do you notice? How would you explain them?

Activity 5: Migration Research
Have students discuss how they think an animal they're studying is influenced by its biological clock. Ask, What actions do you think it triggers on a daily and annual basis? What questions do you have? Give them time to conduct research to check their theories and answer questions raised.

Making Connections

  • Use some of the questions at the end of each activity to inspire discussion, reflection, and journal responses.
  • Have students maintain running lists during the Journey North season of the ways in which migrating animals seem to use internal clocks and why they think this capacity to "tell time" is important. They can also reflect on how internal clocks govern plants and help them survive (e.g., timing of nectar production).


  • Have students choose one Journey North migratory animal to write about and consider what would happen if this animal were to lose track of internal time. Ask them to write stories about the resulting consequences. (Remind them to consider all the activities the animal does according to a daily and annual cycle.)
  • Check that students understand that 1.) biological clocks are internal triggers that allow animals to anticipate and prepare for upcoming events, 2.) these are vital to survival, and 3.) sunlight (especially daylength) is the most powerful trigger for these activities. They should also be able to name several types of activities triggered, at least in part, by biological clocks.

National Science Education Standards

The behavior of individual organisms is influenced by internal cues (such as hunger) and by external cues (such as a change in the environment). Humans and other organisms have senses that help them detect internal and external cues. (K-4)

All organisms must be able to obtain and use resources, grow, reproduce, and maintain stable internal conditions in a constantly changing external environment. (5-8)

An organism's behavior evolves through adaptation to its environment. How a species moves, obtains food, reproduces, and responds to danger are based in the species' evolutionary history. (5-8)