The Internal Clock and
a Sense of Time
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.
1 period each for first 3 activities; 1 week+ for each of last 2 activities
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.
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.
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.
1: Telling Time From the Gut!
the classroom clock and ask students who wear watches to take them
off for the day.
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
- 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.
2: Sunrise, Sunset
- 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.
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.
- 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?
3: Time Awareness
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:
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?
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?
4: Sleep Research
this sleep experiment with students.
the week, students record the number of hours they sleep each night.
- On Friday
each calculates the average for the week.
- 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.)
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?
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.
- Use some
of the questions at the end of each activity to inspire discussion,
reflection, and journal responses.
students maintain running lists during the Journey North season
of the ways in which migrating animals seem to use internal clocks
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).
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.)
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.
Science Education Standards
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)
must be able to obtain and use resources, grow, reproduce, and maintain
stable internal conditions in a constantly changing external environment.
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)