Biological Clocks & Circadian Rhythms

By Robertson, Laura; Jones, M. Gail | Science Scope, February 2009 | Go to article overview

Biological Clocks & Circadian Rhythms

Robertson, Laura, Jones, M. Gail, Science Scope

Every morning when I arrive in my classroom, the leaves of the shamrock plants unfurl; every night at sunset, these same leaves close up. What controls this leaf movement? Can the leaf movement be altered? Why does the leaf close at night? Students can explore these questions and others with this inexpensive houseplant.

Biological clocks are a ubiquitous biological phenomenon that scaffolds the behavior and regulation of all living systems, including middle school students. As students explore biological clocks and circadian rhythms, they are provided with opportunities to connect learning to experiences and observations from their own lives.

The study of biological clocks and circadian rhythms is an excellent way to address the inquiry strand in the National Science Education Standards (NSES) (NRC 1996). Students can study these everyday phenomena by designing experiments, gathering and analyzing data, and generating new experiments. These investigations also address the Standard of teaching unifying concepts and processes though examining form and function, systems, order, and measurement (NRC 1996).


Biological clocks control the circadian rhythms of organisms. The term circadian means "about a day" and is used to describe rhythms that have a 24-hour cycle. The most obvious circadian rhythm in animals is the sleep/wake cycle, but it is only one of many circadian rhythms.


Other examples include daily cycles in blood pressure, body temperature, heart rate, hormone levels, gene expression, and cellular division. Biological clocks have been found in every type of living organism, from bacteria to humans. There is also evidence to suggest that every cell of an organism contains its own clock that influences the functioning of the cell.

Biological clocks are internal, genetic clocks that respond to the environment. However, because they are endogenous (internal), circadian rhythms persist in constant conditions (such as the absence of light). The adaptive value of an internal rhythm is that an organism is able to predict changes in the environment. For instance, birds begin foraging for food in the morning and do not have to wait for the sun to come up. Biological clocks also control circannual rhythms, which have a yearly cycle. Circannual rhythms allow organisms to prepare for seasonal cycles through changes in coat color or migration. The daily movements of shamrock plants include opening their leaves during the day and closing their leaves at night. If this is a true circadian rhythm, the plants will continue to open and close their leaves in constant lighting conditions. Encourage students to locate additional information about plant circadian rhythms with reference materials and the internet. An experiment to test this behavior is outlined below.

Experimenting with shamrocks

The following section outlines a basic setup for observing the circadian rhythms of shamrock plants. This basic setup can be used with inquiry projects designed and completed by small groups of three to four students (see Activity Worksheet). Shamrock plants can be obtained as either bulbs or ornamental houseplants. These naturally occurring plants can sometimes become a pest due to their hardy, quick-growing nature, but that comes in handy for indoor experiments. They are easiest to find around St. Patrick's Day, and plants usually cost $5 or less (they are often on the clearance rack the day after). These plants can be divided into four or more smaller plants for student experiments. Bulbs can be purchased online year round for approximately 30 cents each. If planted as bulbs, small plants with many leaves can be expected in about two weeks. Each small group of students needs one or two shamrock plants, depending on the experiment they conduct. If the project starts with healthy plants, the experiments can be completed in four days. …

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