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Julie Reynolds
 on October 05, 2022

Biological Clocks and God’s Good Creation

For everything there is a season and an internal clock (or two) to keep track of them. Biological clocks can be found in bacteria, insects, birds and even humans.

Side by side images of day and night

On the fourth day of creation God ordered lights to appear in the sky, separated the day from the night, placed the sun and the moon in the sky, established days, seasons, and years, and called it “good.”

God has also provided two biological clocks—circadian clocks and photoperiodic timekeepers—to measure time and provide a way for living things to respond to the passing of days, seasons, and years. The circadian clock tracks the cycle between day and night over the course of a day, and the photoperiodic timekeeper tracks the length of each photoperiod, a term used for the number of hours of daylight during a 24-hour period, and changes in the photoperiod that define the passing of the seasons.

Clocks are not included in the creation narrative, so we don’t know exactly when they first appeared. But scientists think clocks originated more than a hundred million years ago. One of the most ancient clocks is found in cyanobacteria, a primitive one-celled organism. Cyanobacteria have a blue-green pigment that allows them to use sunlight to produce energy. Scientists think that having a way to track and predict when sunlight would be available gave them an advantage over any neighbors who had to just take their chances. Over evolutionary time, the number of organisms with a clock outnumbered the ones that couldn’t respond to predictable changes in the seasons. Today, thanks to natural selection, clocks are found in bacteria, fungus, plants, insects (and other spineless wonders), and vertebrates, including us.

One thing I find surprising, and more than a little amazing, is that even though so many wildly different types of living things have clocks, they all have many of the same basic parts. They all share pigments that detect light, a way to keep track of how long the light is present each day, and a method for knowing whether there was the same amount of light the day before. In insects, the group of animals that I use for my research, the circadian clock includes two interacting feedback loops that, in my opinion, are as intricate and awesome as any human designed timepiece.

Clocks are not included in the creation narrative, so we don’t know exactly when they first appeared. But scientists think clocks originated more than a hundred million years ago.

The mechanics of biological clocks

Proteins with names like Clock, Cycle, Timeless, Period, Clockwork Orange, and Takeout form complexes that work together in several interacting feedback loops. One thing I find really cool is that one complex of proteins, made up of Clock and Cycle, controls production of a second protein complex (Timeless and Period) that, in turn, regulates production of itself by interfering with the on/off switch for the first complex. There is also a light–sensing pigment that degrades the Timeless part of complex 2 when light is present and keeps it from interfering with the first complex. Together these protein complexes create an elaborate, repeated rhythm of protein production and degradation that marks the passing of time.

Cartoon illustration of molecular components that drive biological clocks in insects
During the day, protein complexes in the brain create interacting feedback loops that act as a timekeeping system.  The protein complex (CLK/CYC) binds to DNA and activates production of a second complex (TIM/PER). The TIM/PER complex block CLK/CYC and stop it from producing more TIM and PER. CRY is turned on by light and causes TIM to degrade and keeps the TIM/PER complex from blocking CLK/CYC. Fly graphic created by A Reynolds. Image designed using BioRender Software.

Proteins that make up the circadian clock do more than just regulate themselves. They also interact with clock-output proteins to maintain regular rhythms of activity and rest. Clocks also make sure that other body functions occur according to schedule. The circadian clock also influences activities that need to occur during a certain season. My favorite example is insect diapause, a phenomena that is a focus of my research program at The Ohio State University.

Examples of biological clocks

Diapause is a type of dormancy that is internally regulated by networks of hormones and proteins, including circadian and photoperiodic clocks. Diapause allows insects, and other animals, to survive seasons when food and water are limited, and temperatures are dangerously low. In many ways insect diapause is like hibernation in mammals, frogs, and turtles. Both types of dormancy include changes in energy use, increased protection against hostile environments, and other adaptations that allow them to survive until winter is over and the environment regains enough resources to sustain life.

A key difference between diapause and hibernation is that diapause begins well before winter starts in response to a cue, often a specific number of daylight hours in a 24-hour period, that tells each species that it is time to get ready for a change in the seasons. The amount of daylight is detected and processed by the circadian clock and its interaction with the photoperiodic timekeeper. Together these clocks send a signal from the brain to other parts of the body that, ultimately, leads to changes in physiology, like turning off processes that consume a lot of energy, like growth and reproduction, and ramping up production of protective chemicals that prevent ice formation and limit tissue damage. Scientists don’t completely understand the nature of the relationship between the circadian clock and photoperiodic timekeeper. But laboratory experiments on mosquitos have shown that interfering with proteins in the clock causes females to enter diapause even though their environment indicates that it is a season to remain active.

Junco perched on branch Pouliot ; A Junco perched on branch.

…[biological] clocks are just one more example of God’s good creation and the ways he takes care of even small details…The more I learn about biological clocks, the more I am in awe of these molecular creations and their creator.

Diapause is just one example of a seasonal phenomenon, and insects are just one group of organisms that have thoroughly studied circadian and photoperiodic responses. Another example is seasonal reproduction in birds. In the wild, Juncos (Junco hyemalis) reproduce during seasons with long days. When these birds are kept in a lab environment where a day has less than 12 hours of light, they remain in a “non-reproductive” state even if other conditions, like food availability and temperature, are suitable for successfully raising young. If days are long, they can reproduce even if the temperatures are too cool. Seasonal changes in metabolism, the immune system, and outward appearance have been studied in hundreds of animals, and we now know that they are controlled by the circadian and photoperiodic clocks. There are also seasonal changes in plants, like flower and seed production, which are regulated by the number of hours of light each day.

God’s good creation and our role

Together these clocks are just one more example of God’s good creation and the ways he takes care of even small details. The interacting dance of proteins that make up each clock are amazing, and in my opinion, every bit as awe inspiring – although less picturesque – than the stars in the sky. The more I learn about biological clocks, the more I am in awe of these molecular creations and their creator.

I am also in awe of the tools, like the circadian clock, that he has given to all living things so that they can sense changes in the environment and make necessary changes. However, we need to be mindful of the gift of adaptation, especially when we consider things like global climate change. Historically, changes in the photoperiod have been a reliable indicator of seasonal changes in temperature, water availability, and food quality. Increasing temperatures are disrupting this balance. For example, the beginning of each growing season seems to start earlier each year. Internal systems that once coupled crop flowering time with pollinator activities are no longer in sync. In addition, light pollution disrupts signals that tell birds, and other animals, that it’s time to begin their annual migrations. Nearly constant light in many cities is also disrupting insects’ ability to enter diapause and survive the winter. We might not shed too many tears if mosquitoes are not able to survive, but many butterflies and solitary bees that pollinate crops will also be lost.

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About the author


Julie Reynolds

Dr. Julie A. Reynolds is a Research Scientist at The Ohio State University in the department of Evolution, Ecology, and Organismal Biology. She studies insect physiology and biochemistry with the goal of learning how animals adapt to extreme environments and survive changes in climate. She is a regular contributor for the Emerging Scholars Network Science Corner,  and she is active in discussions about science and faith as a Sinai and Synapses Fellow.