What Is the Circadian Rhythm and Why Do We Have One

What Is the Circadian Rhythm and Why Do We Have One

What Is the Circadian Rhythm and Why Do We Have One

Your circadian rhythm regulates all aspects of your life. And this is not just the case for you, but for virtually all living organisms in the world. Let’s have a look at what the circadian rhythm is and why you have one in the first place. And have you ever wondered what it means for you to live aligned with or against your circadian rhythm? I certainly have. Let’s get started with a quick overview:

What is the circadian rhythm? The circadian rhythm is the natural, internal rhythm our body goes through every single day. It controls and optimizes all our functions and allocates a specific time frame to each. Our circadian rhythm is not exactly 24-hours long and comes from the Latin “circa” (about) and “dies” (day).

Why do we have a circadian rhythm? We have a circadian rhythm because our body cannot perform everything it needs to do at the same time. It optimizes the timing of all our daily functions. And this has been an evolutionary advantage ever since. As a consequence, we are at our best if we align our lives with our circadian rhythm.

Read on to get a full understanding of:

  • What your circadian rhythm is and where it comes from
  • Why it is an evolutionary advantage for you to have a circadian rhythm
  • How the circadian rhythm was discovered
  • Which external factors influence your circadian rhythm
  • My personal experiences and your key takeaways

There is one thing that I can tell you already now: Your circadian rhythm has a bigger impact on you than you might imagine. On every single part of your life. But let’s not get ahead of ourselves.

Evolutionary Advantages

How Evolutionary Advantages Led Us to Develop a Circadian Rhythm

To visualize the importance of your circadian rhythm, think about living outside in nature. No houses, no electricity, no lights, no nothing. Just you and nature. What would help you to survive? How about a body that is well-adapted to your environment

And no matter if your environment is cold or warm, dark or bright, wet or dry – one thing is always true: your environment is regularly and cyclically changing. And the beauty of this is that it allows you to predict its future! Ok, maybe that’s a bit over-promised. But at least you can predict the cyclical parts of your environment and adapt to these – that I can promise!

In short, we have a circadian rhythm because we live in an environment that is cyclically changing, in a predictable way.

So what are those predictable and cyclical environments in nature? I’m glad you asked, here they are:

  • The tidal cycle of 12.5 hours
  • The daily cycle of 24 hours
  • The lunar cycle of 29.5 days
  • The annual cycle of 365.25 days

Now, let’s be a little selfish and here and from now on only look at the daily cycle. Why? Because it is the only one that impacts us. And as promised, you can predict the future of a daily cycle: it goes from day to night and then starts over again. And with this comes predictable environmental changes too.

When you know that something is going to happen, you can prepare for it. Imagine that you will be outside for the next few hours. And you know that it will rain. Would you prepare for the rain? Of course, you would. But what if you knew the weather forecast was wrong and there’ll be sunshine instead? You’d prepare too. But differently, right?

The same is true for your body over the cycle of a day. There are constant and predictable changes in your environment from dusk to dawn and back again. And evolution took notice of these: Throughout every day, your body prepares for a cyclically changing environment. Because it knows a few things that will happen next.

But don’t just take my word for it. Take it from Till Roenneberg, the world’s most prominent chronobiologist (those that study circadian rhythms):​1​

“When organisms are exposed to a regularly changing environment, it is advantageous for them to adapt to these temporal structures, to be prepared for and to anticipate the regular changes. […] The predictive power within temporal structures is an advantage that drove the evolution of biological clocks.”

Till Roenneberg

When you want to know what time of day it is to prepare for the next item on your agenda, you can have a look at any watch. But your body doesn’t have that luxury. So, throughout evolution, it developed a biological clock. And with its biological clock, your body can internally check the time of day to prepare for the next item on its agenda.

Through your biological clock, your body maps its internal time. And through its internal time, your body develops a daily rhythm. However, this daily rhythm of your body is not exactly but only about twenty-four-hours long. To describe this, Franz Halberg​2​ coined the term “circadian” in 1959. Circadian is derived from the Latin circa (about) and dies (day).

In short, your body’s daily rhythm is an about-a-day-long rhythm that is called the circadian rhythm. But how did we get there?

First Discovery

How Was the Circadian Rhythm Discovered

The ability to adapt to the environment has been an evolutionary advantage from early on. This is why biological clocks and circadian rhythms are present in virtually every organism: from us to other mammals and animals, to plants, and right down to those organisms that exist only as one single cell.​3​ (The exception is those that live in places the sun doesn’t reach, like in the deep sea or caves.)

Let’s have a look at the first big leap that led towards discovering the circadian rhythm. What kick-started this monumental discovery? A plant. The mimosa plant to be precise.

In 1729, astronomer Jean-Jacques de Mairan​​​4​ noticed this daily rhythm in his mimosa plant too. While working on his astronomy studies, he observed that his plant’s leaves would follow the sun. And then wondered whether this rhythm would also continue in the darkness. So next, he shielded his plant from the external influence of the sun and put it in constant darkness. (Is this also the first record of procrastination?)

And as it turns out, the mimosa plant does continue its daily rhythm of opening and closing its leaves. In the darkness, without the sun as an external influence. Because of its internal clock aka its circadian rhythm.

This was the first time an experiment demonstrating a circadian rhythm was recorded.

But what happened afterward? Pretty much nothing for about 200 years. But then, firstly botanists, thereafter zoologists, and finally, human physiologists started to investigate circadian rhythms again.​1​

Now, how does the circadian rhythm work for you and your body?

Discovery in Humans

How Was the Circadian Rhythm Within Our Body Discovered

The next big leap was towards gaining insights in the circadian rhythm of our bodies. Yours and mine. It came through an experiment that involved humans and a bunker in the German town of Andechs. But one step at a time.

In the early 1960s, Jürgen von Aschoff and Rütger Wever were determined to find out whether we humans also have a circadian rhythm.​5​ At that time, a circadian rhythm had been shown for many plants and animals. But not for us humans. Not for you and me.

If you had the chance to design such an experiment, how would you do it? Well, it has to involve humans, but in which environment? Would you know which external influences you would have to eliminate?

Back then, Aschoff and colleagues did not know which external influences we use as a time orientation. So they tried to create an environment that eliminates them all.

For their studies, they built two apartments inside the “Andechs Bunker”. With no windows. Completely soundproof. Shielded against any vibrations. And even equipped with a metal cage to keep earth’s electromagnetic field stable. It is safe to say, they went all in.

Over the next two-and-a-half decades, they had about 400 voluntary participants living inside their bunker. One at a time for each apartment. In complete isolation. Lasting from one week to several weeks.

What did they find? Well, no surprise to us today: We humans are governed by an internal clock that produces our circadian rhythm. But there’s more when we look at what happened to these participants:

  1. Their internal days in isolation weren’t exactly twenty-four hours: For most, their internal days were slightly longer, just for a small number they were slightly shorter.
  2. Their body temperature, hormones, electrolytes, and cognitive performance went up and down in synchrony (for most of them)
  3. They usually went to sleep when their body temperature was at the lowest daily point (in real life our body temperature reaches its lowest point only about halfway into our sleep).
  4. Their perception of how fast or slow time passed changed was dependent on the length of their days: For those with longer circadian rhythms, time was perceived to pass slower, and for those with shorter circadian rhythms, time was perceived to pass faster.
  5. But their daily routine continued almost normally with its usual structure: They were awake for two-thirds of their time and asleep for one-third of their time.

Ok, let’s recap. You have an internal rhythm that is called the circadian rhythm. Many functions in your body change according to this rhythm. And your circadian rhythm is most likely longer than twenty-four hours.

So far so good. But want to hear something crazy? For some participants (less than one third), the most extreme day length was twice as long as normal days. Up to fifty hours long! However, those participants never noticed anything different. They ate the same and went to the restroom the same over those extra long days. And they estimated the time it took an hour to pass by about twice as long – correlating with their subjective day length. It was as if their lives went by in slow motion. Crazy, right?

Back to you now. Do you also wonder how that translates to real life? After all, we are all living a twenty-four-hour day. No matter how long our circadian rhythms may be in an experiment. Our external days are always twenty-four-hours long.


Which Environmental Factors Influence Your Circadian Rhythm

You may ask yourself why we did not evolve with an internal clock that is exactly a day length of twenty-four hours long. The answer is that your internal clock relies on external factors from the environment that align it. Those are called zeitgeber (a word coined by von Aschoff that literally translates to “time giver”).

In real life, every environment has a cyclical and predictive part that your body can use to align its internal time. And in our daily cycle, there are two cyclical changes that your body can predict:​1​

  1. Dark and light 
  2. Cold and warm

This is the last puzzle piece in understanding your circadian rhythm. But what do you think, how do these cyclical changes impact your body? Yes! Either through light or temperature. And the correct answer is…

You are mainly impacted by light. Light helps your body to adjust your internal clock to match it with the external day.​6​ Or In fancy words, light is your main “zeitgeber” (If von Aschoff and Wever only had that information before conducting their bunker experiments!).

And for that task, you have one particular set of light receptors in our eyes (they are called melanopsin). They help translate light as your zeitgeber so that your body can align your internal time with the external time. The weird part? These light receptors do not help you see. They are only sensing whether it is night or day. Or anything in between.​7​

Now that this light receptor has sensed what time of day it is for you, what’s next? It sends that information to a small part of your brain that is your central body clock. And your central body clock then regulates many functions of your body. It helps you to perform the right things at the right time.

Find out all about this phenomenon in the post about “How Does Your Circadian Rhythm Work.”

But what does it actually mean for your body to perform the right things at the right time?

Living In Alignment

What Does It Mean to Live Aligned With Your Circadian Rhythm

Every biological function of your body has its specific timing. And your central circadian rhythm optimizes these. In short, your circadian rhythm regulates your physiology, your cognitive functions, and all your major systems like your blood and immune function, your vascular system, and your metabolism.​1​

Ok, that was a high-level overview of what the circadian rhythm regulates. But why does it do all that? Here is what Satchin Panda, another leading chronobiologist, has to say about this:​8​

“Circadian rhythms optimize biological functions. Every function in the body has a specific time because the body cannot accomplish all it needs to do at once”

Satchin Panda

For more details about what it means for your daily life to live aligned with or against your circadian rhythm, check out this post I wrote about “Why Is the Circadian Rhythm so Important for You.”

Random fact: We also used to have a significant circa-annual rhythm. Both daylight and ambient temperatures signaled our bodies what time of year it was. And we had, for example, annual reproductive peaks. But with the beginning of industrialization, we moved from working outside to inside. And when we are inside, we protect ourselves from seasonal fluctuations in both daylight and ambient temperatures. Accordingly, this circa-annual rhythm has become weaker than ever.​9​

Personal Experiences

My Personal Experiences

One of the most direct forms of living against your circadian rhythm that you can experience comes in the form of jet lag. And one of the most beautiful forms of a circadian rhythm comes in the form of tiny bioluminescent single cells glowing in the water at night. Let’s have a look at both.

Jet Lag

My One-Directional Jet Lag

When I fly across different time zones, there are two times my body has to deal with when I arrive in a different time zone. One is the local time of day. And the other one is my internal time – the time my body clock thinks it is based on the local time where I came from.

But I noticed something strange. I have no problems adapting to local time after flying west. But adapting to local time after flying east is difficult for me. No matter how much I try to prepare beforehand.

Why do I have a harder time flying east than flying west? This is because I am generally an evening person. And I learned that this means that my circadian rhythm is longer than twenty-four-hours. Just like for the majority of us and most likely for you too.​5,10​ And when flying west, the external day gets, relatively for me, longer. This is easier for me to adapt because it is more in line with my longer than twenty-four-hour circadian rhythm. And when flying east, it is the opposite.

So, let’s say I crossed three time zones to the west:

  • When landing at noon local time, my body thinks that it is already 3 pm. This adds three additional hours to my day (or as many hours as I crossed time zones).
  • With the morning light as an external zeitgeber (remember, the external influence to align my circadian clock), my body gets signals to adapt to a longer day.
  • But since I am an evening person, I often welcome this change in time.
  • My internal days are longer than twenty-four-hours so that I do welcome that change.
  • I go to bed late according to my internal time but a little earlier than normal according to local time.
  • And the next morning? I am normally wide awake much earlier than normal. And for once feel like a morning person too

But when I fly east? Ok, the same scenario, but the other way round:

  • When landing at noon local time, my body now thinks that it is only 9 am. This takes away three hours from my day (or as many hours as I crossed time zones).
  • I am still an evening person and don’t appreciate that my day is cut short now.
  • My internal time is longer than twenty-four-hours and that makes it even more difficult to fall asleep at an appropriate local time.
  • Mostly I try and fail. Only being able to fall asleep way too late.
  • And my next day(s) will be out of sync.
  • This normally means that I have to eat at times when I am not hungry and when my body is not prepared for it. The same goes for sleep.
  • And my general performance during the day is worse than normal.

By the way, this would be the other way round for you if you have an internal day shorter than twenty-four-hours. So basically if you are a morning person.

My solution? Usage of daylight. Plenty of daylight. Remember how this is your main zeitgeber? This means that this is your main external factor for your body to adjust to external time. For me, that means that I go out into the sun for as long as possible and as early as possible. The stronger this light is, the stronger is its signal for my body to adapt to the local time of day.

Using bright natural light has helped me to align my circadian rhythm with the external time. One of the first things I noticed was that I was able to fall asleep much easier in the evenings. And that I wake up in the morning refreshed, full of energy, and ready to start the day. So my sleep quality must have improved too.

Dennis Kamprad

I can’t say that I do not experience any more symptoms of jet lag any anymore. But since I used this bright natural light hack, the problems of jet lag have minimized. Quite a lot. And I can adapt much quicker to a new local time.


The Bioluminescent Single-Cell

Have you ever heard about bioluminescent marine algae? Those that can illuminate themselves. Those that make it look like the sea glitters at night? And, by the way, they are made of only one single cell.

When I saw such self-illuminating algae on holiday in Vieques, Puerto Rico, I had the feeling that they were performing a light show for me. And it was magical. While I still like to believe that those single-cell organisms performed this beautiful show just for me, their nightly journey is actually driven by their circadian rhythms. And that somewhat feels magical too. For a single cell.

What happens to those marine algae (now called Lingulodinium polyedrum, formerly Gonyaulax polyedra, in case you wondered)? They face a daily dilemma:​11​

  • They need energy, which they can get from the sun through photosynthesis. But only if they stay close to the water surface.
  • And they need nutrients which they get from deeper in the sea. Too far away from the surface to harvest the sun for energy.
  • Their solution to getting both energy and nutrients? They developed an internal clock that makes them go on a nightly journey. Downwards just before the sun sets to harvest nutrients. And upwards just before the sun rises again. All based on their circadian rhythm. Performed with one single cell.

And now to the magical part. When I saw them in shallower waters, I saw them on their nightly journey between energy and nutrients.

To make this journey, they have to stick together. Like a carpet. And when they come into contact with anything, they illuminate. Thanks to their circadian rhythm they went on this nightly journey. And I was able to see an illuminated carpet spanning across parts of the sea.

Key Takeaways

Key Takeaways

Finally, there are five key takeaways that I want to share with you:

  1. Your body is controlled by your biological clock. This clock, however, does not produce a day that is exactly twenty-four-hours long. For most people, it is longer.
  2. This is why your daily rhythm is called the circadian rhythm. And this is also the reason why you have to align your internal biological clock with your external local time.
  3. The biggest external factor to align your biological clock –  and with this your circadian rhythm – is light. Natural light exposure. And you need a lot of it, especially in the first part of the day.
  4. You feel best when you align your daily rhythm with your circadian rhythm. This impacts your daily energy levels, your cognitive and physical performance, your health. And everything else.
  5. You limit your potential in all areas of your life if you live a life against your circadian rhythm. And you make your body more prone to a range of health issues, including chronic diseases.

And now back to you: If you reflect on your typical day, do you already live aligned with your circadian rhythm? If yes, what has helped you on that journey? If no, what would you change?

Stay fit,

PS: If you found this information useful, spread the word and help those who would benefit most from it 🙂



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Hi, I'm Dennis

The content of every post is based on peer-reviewed, published studies combined with my own experience of translating those theories into real-life practice.

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