You might have heard that your biological clock and your circadian rhythm are important for your everyday life. And that there’s a strong connection between those two – but that they are not the same. So, what’s the difference between them? And how are they connected?

What’s the difference between your biological clock and your circadian rhythm? Your biological clock tracks your internal time and your circadian rhythm uses this time information to optimize the timing of your body functions. Think about your biological clock as the watch your body has to track your internal time and the circadian rhythm as the schedule that your body then follows based on this time.

Read on to get a full understanding of:

• What your biological clock is
  • How your biological clock creates its own internal time
  • How a hierarchy of clocks ensures that you have a coordinated internal time
  • How this coordination of internal time works
• What your circadian rhythm is
  • Why the circadian rhythm is so important for you
  • What your circadian rhythm controls in your daily life
• What happens when you disrupt your biological clock and circadian rhythm
• Why our modern environments weaken your circadian rhythm
• My personal experiences and your key takeaways

Ok, let’s start with a little analogy about your biological clock and circadian rhythm to highlight their purpose and how they are connected.

Overview and Differences

What Are Your Biological Clock and Your Circadian Rhythm

Your biological clock and your circadian rhythm are not the same, but they are closely linked. And this link is that your biological clock creates the internal time that your circadian rhythm needs to follow its daily cycle.

• Your biological clock is like your watch: it shows you the internal time of day.
• Your circadian rhythm is like your daily schedule: those are the things that you have to do based on the internal time of day.

And just as you need to know what time of day it is to follow your schedule, your circadian rhythm also needs to know what time of day it is to follow its schedule.

Let’s take one step back for the moment. Why does your body need to follow a schedule in the first place? Simply because there are many functions that your body needs to perform every single day. But your body cannot perform all of those functions at the same time. And some functions only make sense at specific times.^​1​

You can read all about it here in: “Why Is the Circadian Rhythm So Important For You”

Now to the next important question in line: What role plays your circadian rhythm in this? Your circadian rhythm optimizes all your body functions based on your internal time. It is your daily schedule that your body follows. This has been such an evolutionary advantage that virtually all organisms – from single cells to plants to us humans – have a circadian rhythm.^​2​

You can read all about it here in: “Your Circadian Rhythm: How It Controls Your Daily Life”

And, finally: How does your circadian rhythm know about the time of your external day? Here, your biological clock comes into play. It is your internal clock that tracks the time of your external day. And that also makes sure that your internal time matches the time of your external day. At least if you allow it to do so.^​3​

You can read all about it here in: “How Does Your Circadian Rhythm Work: All You Need to Know”

Let me introduce you to a quote from a review paper by leading chronobiologists Till Roenneberg and Martha Merrow aptly titled “Circadian clocks – the fall and rise of physiology,” that summarizes the connection between your biological clock and your circadian rhythm as follows:^​2​

“Molecular processes generate cellular rhythmicity, and cellular clocks in animals coordinate rhythms through interaction (known as coupling). This hierarchy of clocks generates a complex, ∼24-hour temporal programme that is synchronized with the rotation of the Earth. The circadian system ensures anticipation and adaptation to daily environmental changes, and functions on different levels — from gene expression to behaviour.”

Till Roenneberg & Martha Merrow

Ok, you’ve just seen a more detailed version of our previous analogy. So let’s unpack that next. To be more specific, let’s have a look at:

• The cellular rhythmicity, the hierarchy of clocks, and the clock coordination of your biological clock(s). 
• The adaptation to your daily environment thanks to your circadian rhythm.

Biological Clock

What Is Your Biological Clock (aka Your Internal Time)

Your biological clock is like your body’s internal watch. It is your internal time. And this internal time is also the basis for your circadian rhythm. And your internal time is so important for you, nearly every cell of your body has its own biological clock.

Cellular Rhythmicity

How Cellular Rhythmicity Generates Your Biological Clock

How does your biological clock – your body’s internal watch – work? Through your cells producing their own twenty-four-hour cycle. They do this by accumulating and breaking down proteins, in a cycle that takes around twenty-four-hours. To track your internal time.

Cells are able to generate their own twenty-four-hour cycle. And how this cycle works has been a monumental part in understanding how your circadian rhythm works. So monumental that, as recent as in 2017, the Nobel Prize in Physiology or Medicine was awarded for “discoveries of molecular mechanisms controlling the circadian rhythm.”^​4​

And here’s the summary of the work that got awarded the Nobel Prize and that explains how a twenty-four-hour rhythm can be generated on a cellular level:^​5–8​

1. Within cells, there’s the period gene that starts producing the PER protein.
2. Once enough PER gene has accumulated and its levels are high-enough, the PER protein can then bind to another protein (called TIM protein) and reach the cell nucleus to stop the period gene from producing more PER protein.
3. This feedback loop allows it to accumulate the PER protein and then – when there’s enough PER protein – to stop itself being built so that it degrades again.
4. Once the PER protein has degraded and its levels are low-enough again, the period gene can restart the PER gene production.
5. And this cycle – you might have guessed it – takes about twenty-four hours.

Are you still with me? Great! So let’s try to visualize this cycle now with two graphs.

In the first illustration, you can see how protein accumulation and degradation create your cell’s own twenty-four-hour cycle. This cycle starts at the cell nucleus, where the period gene encodes the PER protein until this binds to the TIM protein and then enters the cell nucleus to stop the production of more PER protein. This is a simplified illustration of the molecular components of the circadian clock.

And in the second illustration, you can see how this process happens over the course of day and night. Where the PER protein accumulates during the night and degrades during the day. To create a cellular twenty four hour cycle. This is a simplified illustration of the feedback regulation of the period gene.

• 

• 

This is, simplified speaking, how your biological clock works within your cells. Your cells produce their own twenty-four-hour rhythm through the production and degradation of a protein. And the respective state of this process reflects the biological time of a cell.

But if your cells have their own biological clock, does this mean that they each show different internal times? Luckily not. Because you have a master clock that coordinates all your biological clocks.

Clock Hierarchy

How a Hierarchy of Clocks Ensures a Coordinated Internal Time

So, let me now introduce you to your master clock. The one that rules them all. And the one that coordinates all your biological clocks.

Want to continue with our watch analogy? Sure! Your master clock is like an atomic clock that sends out a signal to align all watches. And your biological clock is like an internal watch that receives this signal to then have the same (coordinated) time as your master clock.

Your master clock – called the SCN – is located in your brain. Just behind your nose bridge, where your optic nerves cross. Why do I tell you this? Because those optic nerves deliver important light information about the time of day to your SCN.^​9,10​

Here’s the thing. Your external day always has twenty-four hours. But your internal biological clock (remember, the process of protein accumulation and degradation) is not necessarily exactly twenty-four-hours long.

And because of this, your circadian rhythm, that follows your biological clock, is (without external information about the time of day) not exactly but only about twenty-four-hours long. This is why this daily rhythm is called “circadian” in the first place, which is derived from the Latin “circa” (about) and “dies” (day) and literally means “about-a-day-long” rhythm.^​11​

Ok, back to the light information that your SCN (your master clock) receives from your eyes through your optic nerves. Because it is through this light information that your SCN aligns your internal time to your external time. It is like a light signal of the time of day. And it helps your SCN to make sure that your biological clocks are exactly twenty-four hours long. This, in turn, ensures that also your circadian rhythm is not just an “about-a-day-long” rhythm but matches your external twenty-four-hour day.

Are you curious to find out more about the crucial importance of light on your circadian rhythm and hence your everyday life? You can read all about it here in: “How Does Your Circadian Rhythm Work: All You Need to Know“

Now, you’ve seen that your master clock aligns with your external day through light information. And that it coordinates your biological clocks in your body. But how does it actually do that?

Clock Coordination

How Your Clock Coordination Works

Let’s take one step back again. And acknowledge that your virtually every body function follows your biological clock. And that this is called your circadian rhythm. And you know what that also includes? Your core body temperature.^​12​

Why is that important? Because your master clock – the SCN – uses your core body temperature to coordinate all your biological clocks. To be more specific, your SCN drives daily fluctuations in your core body temperature. And it is the signal of these relative changes in your body temperature that your biological clocks use to coordinate with your SCN.^​2,13​

Let’s take it from leading researchers Ethan Buhr and colleagues from Northwestern University who published a paper aptly titled “Temperature as a universal resetting cue for mammalian circadian oscillators”:^​13​

“This differential sensitivity to temperature allows the SCN to drive circadian rhythms in body temperature, which can then act as a universal cue for the entrainment of cell-autonomous oscillators throughout the body.”

Ethan Buhr et al.

Ok, let’s unpack this. What you’ve already known is that your SCN – your master clock – drives the circadian rhythm in your body temperature. Now to the other parts of this quote:

• The differential sensitivity to temperature highlights that your biological clocks are differentially sensitive to changes in temperature. Your SCN is not sensitive to temperature differences (but in fact drives them) whereas your biological clocks are sensitive to temperature.
• The universal cue (your body temperature) for entrainment (of your biological clocks) basically means a universal cue for aligning your biological clocks to your SCN. Entrainment in general describes aligning your internal time to a (mostly external cue such as your light exposure).
• And your “cell-autonomous oscillators throughout the body” are your biological clocks throughout your body.

Now, let’s rewrite it to make it clearer: Your master clock (the SCN) drives changes in your body temperature to align all the biological clocks throughout your body.

And when all your biological clocks are aligned by your SCN, then they all show the same internal time. And with an aligned internal time across all cells, you also have one aligned circadian rhythm.

So let’s have a look at your circadian rhythm next.

Circadian Rhythm

What Is Your Circadian Rhythm

Ok, by now we’ve seen that your biological clock tracks your internal time. Or better, your biological clocks throughout your body all track your internal time. And that these are all aligned by your master clock in your brain. So that they all show the same internal time.

Let’s go back to our clock analogy: Just as you have a daily schedule that you follow based on what time it is, your body also follows its daily schedule. This daily schedule of your body is called your circadian rhythm. And it follows your biological clock.

Now,  what does your body do with that knowledge of what (internal) time it is? Or, better, why does your body need its daily schedule – your circadian rhythm? Because through your circadian rhythm, your body optimizes all your functions. This is important for two main reasons:^​1​

• There are many functions that your body needs to perform every single day, but it cannot perform all of those functions at the same time.
• And some functions only make sense at specific times.

And do you want to know how important this daily optimization of all your body functions is for you? Let’s put it in the words of leading researchers Daniel Cardinali and colleagues from the Pontificia Universidad Católica Argentina, Buenos Aires, when they wrote about the “Circadian Organization”:^​14​

“The circadian clock is one of the most indispensable biological functions”

Daniel Cardinali et al.

But there’s probably no better way to show this importance than to refer back to evolution. Why? Because virtually every organism – from single cells to plants to all other animals to us – has a circadian rhythm. The only exceptions are those that live in places the sun doesn’t reach, like in the deep sea or caves.

Now, what does that mean for your daily life? Leading chronobiologists Till Roenneberg and Martha Merrow explained this best when they stated:^​2​

“The circadian system ensures anticipation and adaptation to daily environmental changes, and functions on different levels — from gene expression to behaviour.”

Till Roenneberg & Martha Merrow

And here’s what your circadian rhythm controls in your daily life. You can view this list as an overview of the schedules that your circadian has control over. Based on the input it receives from your biological clock:

• Your sleep-wake-cycle (aka your chronotype)
• Your physical performance
• Your cognitive performance
• Your mood fluctuations
• The function of your organs
• The function of your immune system
• The efficacy of the drugs you take
• How much jet lag you experience

Want to find out all the details about each of these? Then I can highly recommend you to check out the following article: “Your Circadian Rhythm: How It Controls Your Daily Life”

For now, let’s have a little closer look at your sleep-wake cycle, which is the most direct expression of your circadian rhythm. Or at least the one, where you feel it most. And this expression of your sleep-wake-cycle is called your chronotype.^​15​

Now, you can view your chronotype as the one in charge of the timely details of your daily schedule. Let me explain.

You have seen that your body needs to optimize the timing of all its functions. But for some people that happens earlier. While for others this happens later. This is where your chronotype comes into play. Simplified speaking, it determines the exact timing of all your body functions. And with this also if you prefer to wake up early or go to bed late (or anything in between).

Do you want to find out all about your chronotype? Check out this article about “What Are Chronotypes and How to Find Out Yours”

Clock and Rhythm Disruptions

How a Disrupted Biological Clock Leads to a Disrupted Circadian Rhythm

By now you should have seen how important it is for you that your biological clocks show the right time. To allow your circadian rhythm to optimize your body functions based on the right time of day.

But it might be even easier to show examples of what happens when this is not the case anymore. And we are having a look at three of these next.

Master Clock Significance

How Your Body Loses Its Sense of Time Without Your Master Clock

Now, what happens when your biological clock doesn’t work anymore? Well, your body would lose its coordinated sense of time.

Let’s take patients with Alzheimer’s disease in their later stages as an example of that. What happens with this disease is that the brain degenerates. And in later stages, also the part of the brain with your master clock (your SCN that coordinates all your biological clocks) degenerates.^​16,17​

What then happens to those patients is that their body loses their sense of time. The most obvious issue is that they have great problems keeping a regular sleep-wake cycle. But the timing of everything else that is connected to their biological clock is also disrupted. And their body cannot optimize the timing of its functions anymore. Which leads to more problems across all aspects of their daily life.

Jet Lag

Why Jet Lag Is a Problem of Misaligned Biological Clocks

So, your biological clocks are not aligned if they cannot be coordinated by your master clock. But what happens if they are coordinated to the wrong time? Let me introduce you to jet lag.

You experience jet lag when your biological clocks show a different internal time than your external time. And, as a result, your body optimizes its functions based on an internal time that doesn’t match your environment anymore. And it should be clear by now that jet lag is more than a sleep problem. That’s why its symptoms don’t just disappear after a good night’s sleep. The solution? Align your biological clocks to your new external time and jet lag is gone.^​18,19​

To align your biological clocks to a new external time is easier said than done. But there are proven ways to do so. You can find out all about it here in: “Use Your Circadian Rhythm to Beat Jet Lag: The Ultimate Guide”

Modern Environments

Modern Environments

How Our Modern Environments Weaken Your Circadian Rhythm

Last, but definitely not least, our modern environments disrupt your biological clocks on a daily basis. And that weakens (or even disrupts) your circadian rhythm.

Do you remember that your master clock – your SCN – needs light as an environmental cue to align your internal time to your external time? And this is where the problem lies with our modern environments.

Throughout evolution, we used to live and work outside, exposed to sunlight all day long. And our master clock has used natural light information to align to the external (sun) time. Now, sunlight is crazy intensive during the day and then goes away in the evening and is fully gone at night. At the same time, the sun’s light rays are high in blue colors in the first part of the day and then high in amber colors for the second part of the day.^​3,20​

Now, that’s also the light information that your SCN needs to align to your environment.

But then, things changed. With the start of the industrial revolution in the 1760s, we started to work indoors. And moved away from getting the natural light exposure during the day. Then electrical light increased out light exposure in the evening and at night. And, with the advent of white LED-lights as recent as the 1990s, those lights became in blue rays. And, finally, all your screens also emit too high levels of blue light rays (at least for your SCN).

Here’s what that means for your SCN, your master clock:

• You don’t get enough light during the day.
• You get too much light & blue light during the evening and night.

The problem then is that your SCN gets weakened with that light information. Let me show you two of the main problems you experience because of that:^​1​

1. Your SCN expects crazy high amounts of blue-light rays in the morning but doesn’t get them. 
   • As a consequence, your circadian rhythm doesn’t fully shut down your night activities and doesn’t fully ramp up morning activities.
   • How does that feel to you? You are both tired and not energized when you wake up.
2. Your SCN doesn’t expect high amounts of light and especially no more blue light rays during the evening and at night. But it gets too much of both of them. 
   • As a consequence, your circadian rhythm doesn’t fully shut down your day activities and doesn’t fully ramp up your night activities.
   • How does that feel to you? You are both still too energized to go to sleep and have trouble getting restorative sleep.

Do you remember that your circadian rhythm optimizes the timing of all your body functions? And that it does so based on your internal time it gets from your SCN, your master clock? Well, with the wrong light information, you’d weaken (or even disrupt) your SCN. And your circadian rhythm wouldn’t be able to optimize the timing of all your body functions anymore. Just like in the one example above. But for virtually everything in your body.

Do you want to find out more about how you can strengthen your circadian rhythm? Then I highly recommend you to check out this guide: “Get Your Circadian Rhythm Back on Track: The Ultimate Guide”

Personal Experiences

My Personal Experiences

When it comes to the connection between my biological clock and my circadian rhythm, there is one thing that I want to highlight again: my daily light exposure.

Why? Because a few small changes have made a noticeable difference for me to align my biological clocks and to ultimately strengthen my circadian rhythm.

Here’s what I do to increase my light exposure during the day:

• Directly after waking up, I do some light exercise outside. On most days this is either an easy run or simply walking for a few minutes around the block.
• During the day, I’m quite strict about getting lots of light. Preferably natural sunlight. This could be through working directly at a window or even outside. But if both are not possible then I use light showers for my desk. Actually two, one on each side of my laptop.

And here’s what I do to reduce my exposure to artificial (blue) light at night in the evening/ at night:

• All my screens have an automatic night mode enabled, where they reduce their blue-light spectrum, I have my lights set at warmer tones, and I change my glasses to ones with a blue-light filter.
• And, most importantly, I limit my screen time.

And what is the additional time of these changes? The only thing that really adds here is the morning exercise. Everything else just goes along what I’d be doing anyway. Minus the screen time in the late evening, that actually gives me time back.

Why am I telling you about this? Because of the positive impact that I can feel every single day. Through these little changes that are relatively easy to implement. And, more importantly, because I do hope that you try some of them out as well and have the same positive experiences that I have.

Key Takeaways

Key Takeaways

Finally, there are three key takeaways that I want to share with you about your biological clock and your circadian rhythm:

1. Your biological clock is like your watch: it tells your body the (internal) time of day.
   • Your cells create their own nearly twenty-four-hour rhythm: They achieve this through a feedback loop that is about twenty-four-hours long. This feedback loop involves the accumulation and degradation of a protein and its discovery was awarded a Nobel Prize. The respective state of this process reflects the biological time of a cell.
   • Your master clock – the SCN – is like an atomic clock that aligns all your biological clocks: And it uses external light information to make sure that all your biological clocks throughout your body are exactly twenty-four-hours long.
   • Your SCN coordinates all your body clocks through changes in core body temperature: Your biological clocks are sensitive to temperature changes and use the signal of relative changes in your body temperature to coordinate their internal time with your SCN.
2. Your circadian rhythm is like your body’s daily schedule: those are the things that your body does based on the time of day.
   • Your circadian rhythm optimizes the timing of all your daily body functions: There are many functions that your body needs to perform every single day, but it cannot perform all of those functions at the same time. And some functions only make sense at specific times.
   • Your circadian rhythm controls virtually everything: From your sleep-wake-cycle (aka your chronotype), your physical performance, your cognitive performance, your mood fluctuations, the function of your organs, the function of your immune system, the efficacy of the drugs you take, to how much jet lag you experience.
   • Your chronotype determines the exact timing of all your body functions: For some people, this means that their daily schedule happens earlier, and for some, this means that it happens later. And your chronotype also determines if you prefer to wake up early or go to bed late (or anything in between).
3. Disruptions of your biological clock and your circadian rhythm affect all parts of your daily life.
   • Without your master clock, you would lose your sense of time: The most obvious issue is that you would then have great problems keeping a regular sleep-wake cycle. In general, your body wouldn’t be able to optimize the timing of its functions anymore. This would lead to problems across all aspects of your daily life.
   • You most likely have a weakened circadian rhythm thanks to our modern environments: You don’t get enough natural light exposure during the day and too much artificial light in the evening and at night. As a result, your circadian rhythm isn’t able to optimize the timing of your body functions anymore. The most obvious sign for that is that you still feel too tired and not energized enough in the mornings when you have to wake up but then at night, you are still too energized and your body isn’t ready yet for a night of fully restorative sleep.

And now back to you: Have you discovered the connection between your biological clock and your circadian rhythm in your daily life yet? Or better, have you noticed the differences between times when you naturally have a strong circadian rhythm (often experienced on holidays in nature) and other times when you have a weak or disrupted circadian rhythm (e.g., through staying indoors during the day and getting too much artificial light at night)?

Stay fit,

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

References

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