What is the science behind circadian rhythms?

Spring Forward, Fall Back

This past Sunday, those of us who observe Daylight Savings Time were gifted with an extra hour.  I’m a traditionalist and spent that “extra” hour sleeping. I know others who prefer to stay on their original sleep schedule, using that extra hour to be productive.

No matter how you chose to spend that extra hour, I’m almost certain that you, like so many others, have found yourselves confused in the days following the time change.  As we seem to have collectively all lost our ability to intuit the time, a common refrain has been, “Wait: what time is it?”

How does my dog know it's dinner time?
How does my dog know when it’s time to be fed?

Of all my family members, my dog seems to be the one struggling the most with the time change.  Her internal clock tells her when it’s dinner time, regardless of what the wall clock may say.

Starting about 6:00 pm (5:00 pm since the time change), she follows us around whining and begging for food.  

But how does she know it’s dinner time?  

This has gotten me thinking of our internal biological clocks, more commonly known as circadian rhythms.  

What is a Circadian Rhythm?

The word “circadian” is derived from the Latin roots circa meaning around and diem meaning day. Therefore, the literal translation of circadian is “around a day” or “around 24 hours”.  

In general, the term circadian rhythm is used to describe the internal clock that many living things possess (including humans, animals, plants, and even bacteria) which are roughly synced to a 24 hour hour day.  

What is the Function of a Circadian Rhythm?

Even honeybees have internal biological clocks.
Diurnal animals are awake during the daytime and asleep at night

All of us are aware of circadian rhythms at least to some degree.  

Most of us follow a pattern of wakefulness in the daytime and sleeping at night common throughout the natural world.  Many animals (so-called diurnal animals including dogs, songbirds, deer, squirrels, honeybees, and butterflies) follow this same pattern. Other animals— including raccoons, bats, opossums, skunks, and fireflies—are nocturnal. They have a 24 hour circadian rhythm too— it just operates on a different schedule than ours.  

Studies have shown that circadian rhythms can influence many biological processes including sleep-wake cycles, hormone release, hunger cues,  digestion, body temperature, and much more.

But how do these circadian rhythms get set?

What is the Relationship Between Sunlight and Circadian Rhythms?

The suprachiasmatic nucleus (SCN for short) is the master clock

For as long as people have recognized the operation of these biological clocks, they have known that  circadian rhythms are somehow regulated by sunlight.  

In vertebrates (including humans), a region of the brain has been identified as the “master clock” that coordinates the body’s circadian rhythms.  The suprachiasmatic nucleus (or SCN) is located in the brain hypothalamus and is connected to the optic nerve. When light is received through the eye, a message is relayed through the optic nerve to the SCN.  This explains how the “message” of daylight (or darkness) is transmitted to the brain to control our biological clock.

Learn more about how SCN regulates our body systems in this video:

Circadian Rhythms Take Place at a Cellular Level

As a molecular biologist, I was fascinated to learn that circadian rhythms are regulated at the cellular level. The 2017 Nobel Prize in Physiology or Medicine was awarded to three scientists—Jeffrey Hall and Michael Rosbash of Brandeis University and Michael Young of The Rockefeller University—for their research determining how several genes work together to control our biological clock.

Within each cell, the production of certain proteins (called transcription factors) increase then decrease over the course of a 24 hour day.  Some of these proteins turn ON expression of cellular genes while other transcription factors turn the expression of other genes OFF.  Ultimately, it is the regulation of cellular gene expression by these transcription factors which drives our biological clock— telling each cell what hormones or enzymes to produce, when to repair or replicate DNA, when to use energy, and when to rest.  Every cell in your body has its own clock.

The SCN in the brain is the body’s master clock and makes sure that all of the cellular clocks stay synchronized with each other and with the rising and setting of the sun. 

This helps explain why we may feel confused or disoriented after the Daylight Savings Time time change:  it takes time for our body to adapt to our new schedule.  Every cell in our body needs to sync to the new time. 

It also explains how jet lag or pulling an all-nighter can really leave us feeling horrible.  Our bodies are programmed to operate on a 24 hour schedule down to the cellular level.  I think that’s amazing!

Is Being an “Early Riser” or a “Night Owl” Genetically-programmed?

On an interesting and somewhat related note, other studies have shown that there is actually a genetic component which determines whether or not one is  an early riser (a lark) or a night owl.  This study makes me particularly  happy, as I now have a rebuttal when my husband teases me for liking to sleep in late:  “It’s not my fault! It’s genetic!”

Additional Topics Related to Circadian Rhythms

There are so many interesting questions related to circadian rhythms to consider:  

  • How are the  circadian rhythms of people living near the Earth’s poles affected that experience 24 hours of sunlight (or 24 hours of darkness)?  
  • What are the impacts on people who work night shifts or alternate between day and night shifts?  What happens to their biological clocks?
  • Does artificial light play any role in the regulation of our circadian rhythm (as well as the rhythms of plants and animals exposed to city lights all night long)?

I hope you come away from this post with a deeper appreciation for how amazing your body is—down to the cellular level!

Additional Resources for Learning More About Circadian Rhythm and Daylight Savings Time

Here’s a link to an article that discusses the history of Daylight Savings Time and how it has “evolved” through the years:  Why do we have daylight saving time? 100 years of history

Further Information:

Here are additional resources I used to compose this post:

How the Body’s Trillions of Clocks Keep Time

Clock Genes and Altered Sleep–Wake Rhythms: Their Role in the Development of Psychiatric Disorders

Circadian Rhythms Influence Treatment Effects

Daylight Saving Time Messes With Your Body Clock

The Science of Spring

To learn more about the function of DNA and how genes are expressed, you may be interested in taking my course:  DNA: The Alphabet of Life.  

DNA: The Alphabet of Life self-paced online course

Students will learn about the structure of DNA and its location in living cells.  They will learn how DNA is organized into discrete units of heredity called genes, and learn how genes are expressed through the processes of transcription and translation.  Finally, they will learn how DNA mutations occur and will discover how mutations in cellular DNA can lead to diseases (such as Sickle Cell Anemia, Cystic Fibrosis, Tay-Sachs disease, and cancer).  Periodic quizzes (fact checks) will test students’ understanding before moving forward.  Many activities are provided to give students practice in DNA base-pairing rules, transcription, and using the codon table to translate mRNA into protein.  Directions for 2 labs are included which can each be performed using household materials.

What is a circadian rhythm? How are our circadian rhythms controlled by sunlight?  Why is Daylight Savings Time so hard on our sleep cycle?  Do plants and animals have circadian rhythms?

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