You are probably here because you’ve asked yourself how a “free-running” circadian rhythm fits in the context of your circadian rhythm. Or what the difference between these two is. If so, then you’ve come to the right place. If not, then you’ll still get the full picture of that differentiation. Plus, what this means for your daily life, and how you can use it to your advantage. But first things first.

What is a free-running circadian rhythm? A free-running circadian rhythm is the length of your circadian rhythm that your body would follow if there was no environmental information about the time of day. Think about living in a bunker or cave with no change in light and no clock that you follow to eat or move.

How long is a free-running circadian rhythm? A free-running circadian rhythm is on average slightly longer than twenty-four hours – about 24.18 hours to be precise. The length of a free-running circadian rhythm follows a natural distribution: A few are less than 24h long, a tiny fraction are exactly 24 long, and many more are longer than 24h.

Read on to get a full understanding of:

• What a free-running circadian rhythm and what it means to have one
• Which three environmental cues align your (free-running) circadian rhythm to your twenty-four-hour day
• How cave and bunker experiments paved the way to our current understanding of your (free-running) circadian rhythm
• How long the free-running circadian rhythm is on average and how to find out the length of yours
• Which steps you can take to strengthen and align your (free-running) circadian rhythm
• My personal experiences and your key takeaways

Let’s start with the basics: The word circadian is derived from the Latin “circa” (about) and “dies” (day). Franz Halberg coined this term in 1959 because the daily rhythm of your body – your circadian rhythm – is not exactly but only about twenty-four-hours long. At least when it’s free-running and not aligned through environmental cues about the time of day.^​1,2​

Free-Running CR Highlights

Why Do You Have a Free-Running Circadian Rhythm

Your body has its own daily rhythm that is called the circadian rhythm. And this circadian rhythm helps to optimize the timing of all your body functions throughout the day, which is important, because:^​3​

• Your body cannot perform all its functions at the same time.
• And some functions only make sense at specific times.

Now, your body uses environmental cues (called zeitgbeber; a German word that literally translates to “time-giver”) to make sure that your circadian rhythm matches your external exactly twenty-four-hour long day.

Without your environmental cues (we’ll have a look at them in a bit), your circadian rhythm would most likely not be exactly twenty-four-hours long, but slightly longer. And this state, where there are no environmental cues, is called free-running. In short, a free-running circadian rhythm is a circadian rhythm not exposed to any environmental cues that would allow it to align to the external time of day.^​4,5​

To have a look at some examples of free-running circadian rhythms – including yours – we need to have a look at those environmental cues (called zeitgeber) first. And don’t worry, there’s only three of them.

Zeitgebers (Environmental Cues)

Which Environmental Cues (Zeitgebers) Do You Need to Align Your Free-Running Circadian Rhythm

You’ve seen that your circadian rhythm would be “free-running” (following its own internal length) without your zeitgebers – your environmental cues.

But what are those zeitgebers – those environmental cues – that align your circadian rhythm to your external twenty-four-hour day?

1. The by far most powerful zeitgeber is your daily light exposure.
2. Followed by the timings of your food intake.
3. And then the timings of your exercise and movement.

Let’s have a quick look at each of those now.

Light is the by far most powerful and important zeitgeber for your circadian rhythm. In fact, light controls your central circadian rhythm, which then orchestrates all other circadian rhythms in your body.^​1,6,7​

Want to know more about light as your zeitgeber? Check out this article: “How Does Your Circadian Rhythm Work: All You Need to Know”

Just like light as a zeitgeber impacts your central circadian rhythm, food as a zeitgeber especially impacts the circadian rhythm of your organs. To be more specific, any calories consumed impact the circadian rhythm of your organs.^​8​

Want to know what that specifically means for you? Check out this article: “When Is the Best Time to Eat – Based on Your Circadian Rhythm”

Your exercise and movement timings act as a distant third place as a zeitgeber. But their timing still has a powerful effect on you. On the one hand, you can use exercise to strengthen the circadian rhythm of your muscles.  And it turns out exercise during both the morning and the late afternoon/ early evening is especially beneficial for your musculoskeletal health (including maintenance and function).^​9–11​

Want to know about the best timing to exercise for your specific purpose? Check out this article: “When Is the Best Time to Exercise – Based on Your Circadian Rhythm”

Unfortunately, one thing that aligns all your zeitgebers, is that they have become too weak and their timing is off in our modern environments.

• We don’t nearly get enough natural light during the day and too much artificial light at night.
• We eat for too long and/ or too late.
• We don’t nearly move enough anymore.

Now, does this mean that your circadian rhythm in our modern environments is closer in length to your free-running circadian rhythm? Well, first and foremost, it means that our modern environments weaken your circadian rhythm. And, in most cases, they make it longer than your twenty-four hour day.^​3​

The result? Your circadian rhythm cannot optimize the timing of your body functions anymore. And that leads to reduced performance and increased health risks (among other things).

Find out all the details about this and how to overcome it in this article: “How to Get Your Circadian Rhythm Back on Track: The Ultimate Guide”

Ok, let’s go back to the free-running circadian rhythm again. To understand the role of our zeitgebers (environmental) a little bit better, we need to have a look at the original experiments that led to its discovery. Not too long ago in the last century.

Zeitgeber-Free Experiments

How Eliminating All Environmental Cues (Zeitgebers) Led to the Discovery of the Free-Running Circadian Rhythm

Two fundamental experiments helped us understand our free-running circadian rhythm. The first one happened in 1938, deep in the Mammoth Cave in Kentucky. And the second one started in the 1960s, spanned for two-and-a-half decades, and happened in a bunker in Bavaria, Germany.

And because those researchers didn’t know about the three most important zeitgebers (that name wasn’t even coined yet), they tried to shield themselves and their participants against any and all of them. Things could have been much simpler if they only knew back then when you and I know now…

In 1938, Nathaniel Kleitman, a sleep researcher at the University of Chicago, and Bruce Richardson, his graduate student, went into the Mammoth Cave of Kentucky for thirty-two days. A location, where there was “no difference between day and night.”^​12​

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The aim was to try to adapt to a new day-length of twenty-eight hours. And while the younger Richardson adapted to this new sleep-wake pattern within a week, the twenty-year-older Kleitman failed to adapt even after more than a month.

The main outcome of this experiment was that our bodies maintain an about twenty-four-hour cycle (our circadian rhythm) even with the absence of external cues (called free-running).

Then, in the 1960s, Jürgen von Aschoff and Rütger Wever were determined to build on this experiment and find out more about our free-running circadian rhythm.^​13​

So they created an environment that eliminated all possible external factors that could act as environmental cues to influence our circadian rhythm. They built two apartments in a bunker, located inside a hill. This place was called the “Andechs Bunker.”

And the interior of those “Andechs Bunker” was shielded against any environmental cues that could reveal information about the time of day to their participants: It had no windows, was completely soundproof, protected against vibrations caused by even the heaviest vehicles on nearby roads, and was also equipped with a metal cage to keep out the more-or-less regular changes of the earth’s electromagnetic field.

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

What did those decades of research reveal? First, participants have a free-running circadian rhythm (a daily rhythm in the absence of environmental cues to reveal the time-of-day information). But one that is not exactly twenty-four hours long:

• For most volunteers, free-running circadian rhythms were slightly longer than twenty-four hours.
• Just for a small number of participants, free-running circadian rhythms were shorter than twenty-four hours.

Ok, but how long are our free-running circadian rhythms then?

Free-Running CR Length

How Long Is the Free-Running Circadian Rhythm

When it comes to the length of the free-running circadian rhythm, there are two types of views that we should take:

1. How long is the free-running circadian rhythm on average?
2. How long is your free-running circadian rhythm and how do you compare with the average?

So let’s have a look at these two different, yet related, views next.

Average Length

What Is the Average Length of a Free-Running Circadian Rhythm

You’ve seen above, from the “Andechs Bunker” experiments in the 1960s and beyond, that the free-running circadian rhythm is on average slightly longer than twenty-four hours.

Let’s have a look at a later study that puts a number behind it.

In their monumental 1999 paper, Harvard professor Charles Czeisler and colleagues put a number behind the “slightly longer than twenty-four-hours average.” Based on their studies, our free-running circadian rhythm is on average 24.18 hours long.^​5​

So much about how we found out about our circadian rhythms and how long they’d be without any environmental cues.

But I guess you are wondering now, how long your free-running circadian rhythm would be. So let’s have a look at exactly this next.

Length of Your CR

How Long Is Your Free-Running Circadian Rhythm

When it comes to finding out more about the length of your free-running circadian rhythm, we can’t exactly put you in an experimental setting without any environmental cues. And I guess you wouldn’t be interested in those rather theoretical findings either.

What you would be more interested in finding out is how the length of your free-running circadian rhythm impacts your everyday life. And, for that, we can have a look at some of your everyday data. The result won’t be an exact number, but a relative length. Relative to your normal day.

First, what is the clearest expression of your circadian rhythm in your everyday life? It’s our sleep-wake cycle.^​3,14​

And when you have a look at your sleep-wake cycle, your circadian rhythm plays a key role in determining when you are able to fall asleep. While your alarm clock in most cases plays the key role in waking you up the next morning.

Now, the expression of your circadian rhythm through your sleep-wake cycle is called your chronotype.

And the connection between your chronotype and your free-running circadian rhythm is as follows:^​14​

1. If your free-running circadian rhythm is longer than twenty-four hours then you are generally a late chronotype. Just like the majority.
2. If your free-running circadian rhythm is shorter than twenty-four hours then you are generally an early chronotype.

With that in mind, let’s now have a look at your chronotype so that we can figure out the relative length of your free-running circadian rhythm. And the scientifically established way to find out your chronotype is to calculate what is called your midsleep point.

Your midsleep point is simply the point in the middle between when you fall asleep and when you wake up. The big advantage of using this point is that it is independent of your sleep need. And, yes, also the sleep need varies among people – despite the commonly cited 8 hours needed.

One more thing: Only use those days when you don’t use an alarm clock to wake up (normally the weekends). Because only those days reflect when your circadian rhythm nudges you to wake up – instead of an alarm that forces you to do so.

And this is how you can calculate your midsleep point (aka your chronotype) to figure out the relative length of your free-running circadian rhythm:

1. Write down when you fall asleep.
2. Write down when you wake up.
3. Take the middle point between when you fell asleep and when you woke up.

Easy, right?

Want to find out the full story about chronotypes? Sure, check out this article: “What Are Chronotypes and How to Find Out Yours”

Now, what do we do with your result? We compare it with the best scientific data we have.

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In the first graph, you can see the statistical distribution of chronotypes in the population. The most common midsleep points are between 4 am and 5 am.

In the second graph, you can see how sleep varies for different midsleep points during the week (black dots) and on the weekend (white dots). And this is where we can find out the relative length of your free-running circadian rhythm.

The thing is that sleep need is an average of the sleep you need both during the week and on weekends. And if you don’t get enough sleep during the week, then you simply need to repay that sleep debt on your weekends.

Let’s be super concrete now. This is what your midpoint of sleep means for the duration of your free-running circadian rhythm:

1. You have a midsleep point (chronotype) later than 3 am: You get not enough sleep on your workdays because you can’t fall asleep early enough. Your alarm clock the next morning cuts down on your sleep time and you have to catch up through more sleep on your free days. This is the case if your free-running circadian rhythm is longer than twenty-four hours.
2. You have a midsleep point (chronotype) around 3 am: You get enough sleep on both your workdays and your free days. This is the case if your free-running circadian rhythm is twenty-four hours long.
3. You have a midsleep point (chronotype) earlier than 3 am: You go get enough sleep on your workdays. But you don’t get enough sleep on your free days. Your social pressures don’t let you fall asleep early enough until you naturally (thanks to your circadian rhythm) wake up early the next morning. This is the case if your free-running circadian rhythm is shorter than twenty-four hours.

This condition of catching up on sleep on the weekends is called social jet lag. You can read all about it here in this article: “What Is Social Jet Lag and How to Overcome It”

Let’s connect this information about the length of your free-running circadian rhythm now with your zeitgebers (the environmental cues) to see how you can use them together for your benefit.

Free-Running CR Alignment

How to Align Your Free-Running Circadian Rhythm to Your External Day

Do you remember the three zeitgebers that your body uses to align your circadian rhythm to your external always-twenty-four-hours-long day?

The one zeitgeber that rules them all is your light exposure. This is because you have an internal master clock, which controls your circadian rhythm, and aligns it throughout your body.

Your body uses external light information to align your circadian rhythm. And here is how this works:

• Light in the first half of your day speeds up your circadian rhythm. Your body recognizes that it is already daytime and speeds up its processes. And the earlier in the morning you are exposed to (natural) light, the more this process happens. This makes it effectively shorter.
• Light during midday has little or no effect on your circadian rhythm. This is no new information for your body: it expects light and there is light. Accordingly, it does not need to adapt your circadian rhythm.
• Light during the second half of your day slows down your circadian rhythm. As long as there is light, your body thinks that it still has to be daytime and slows down its processes. And the later in the evening you are exposed to (artificial) light, the more your body slows down your circadian rhythm. This makes it effectively longer.

You can see the whole process outlined in this article: “How Does Your Circadian Rhythm Work: All You Need to Know”

I guess you’ve already seen two of the biggest problems here. And both are connected to our modern environments:

• You don’t nearly get enough natural light exposure during the day, because you stay most of the day indoors.
• You get too much artificial light exposure in the evenings and at night, because of all the screens and lamps (especially LEDs) you use.

Both of these problems are the main culprit that weaken and disrupt your circadian rhythm. And make it effective longer. Which, in turn, gives you trouble falling asleep during the week.

Want to know how you can overcome these and the other most common problems? So that you can strengthen and align instead of weaken and disrupt your circadian rhythm? Find out all about it in this post: “Get Your Circadian Rhythm Back on Track: The Ultimate Guide”

Personal Experiences

My Personal Experiences

I know that my free-running circadian rhythm is slightly longer than twenty-four hours because my midsleep point would normally fall after 3 am. But I also know that I can heavily influence that through my daily light exposure.

What does this specifically mean?

• I try to get as much natural light during the day as possible. Normally, I start my day with a short walk or run outside. And if I can’t work outside or next to a window, then I use those light-shower lamps that let my brain think that I’m actually outside.
• I try to limit my artificial light exposure in the evening and at night as much as possible. That includes everything that emits blue-light rays (since those are especially important), such as screens and LEDs. And I also use blue-light-blocking glasses.

Especially the use of light as my main zeitgeber has helped me to align my circadian rhythm to my day.

And where I can feel it the most is that my body is ready to sleep in the evening. My sleep quality has improved a lot too (it also helps that I stop eating early enough in the day). And I don’t need an alarm clock to wake up in the morning anymore. To wake up fresh, relaxed, and full of energy to start the day.

Why am I sharing this with you? Because I believe that understanding your (free-running) circadian rhythm is but the first part. Even more important is how you use that knowledge to your advantage. And for this, I’m more than happy to highlight the latest scientific findings and what my personal experiences are with implementing that science.

Key Takeaways

Key Takeaways

Finally, there are five key takeaways that I want to share with you about the free-running circadian rhythm:

1. Your circadian rhythm is entrained (read: aligned) to your external day through zeitgebers (read: environmental cues) with information about the time of day.
   • The by far most powerful zeitgeber is your daily light exposure.
   • Followed by the timings of your food intake.
   • And then the timings of your exercise and movement.
2. Your body follows a free-running circadian rhythm only in the absence of time of day information that it normally receives through zeitgeber. This was first discovered through a cave study in 1938 and further established through a bunker experiment starting in the 1960s and spanning over two-and-a-half decades.
3. Most free-running circadian rhythms are (slightly) longer than twenty-four hours, with the pointing toward 24.18 hours. However, some free-running circadian rhythms are (slightly) shorter than twenty-four hours and a tiny fraction is exactly twenty-four-hours long.
4. You can find out the relative length of your free-running circadian rhythm through having a look at its most direct expression: your sleep-wake cycle.
   • Find the midpoint of sleep (average of when you go to sleep and when you wake up) for the nights when you can freely fall asleep and wake up.
   • Your free-running circadian rhythm is likely twenty-four-hours long if this is at 3 am, longer if it is later than 3 am (in the majority of cases), and shorter if it’s earlier than 3 am.
5. You can align your (free-running) circadian rhythm better to your external day by manipulating your daily light exposure. For the vast majority, this means to get as much (natural) light during the day and as little (artificial) light in the evening and at night to strengthen and align your circadian rhythm.

And now back to you: Have you already figured out how long (relatively) your free-running circadian rhythm would be? And now that you know about your free-running circadian rhythm and what that means for your daily life, do you already have concrete plans to align and strengthen it (especially through the right light exposure)?

Stay fit,

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

References

References

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