The study of the circadian rhythm (chronobiology) is still developing. And this means that it is also constantly evolving based on the latest research insights. But this also means that a few of the previous findings are now outdated and replaced. And this is also the reason why there is still some confusion about the length of our circadian rhythms. Let’s get into this part here.

How long is your circadian rhythm? On average, our circadian rhythm is slightly longer than twenty-four hours, with one major study pointing towards 24.18 hours. The length of our circadian rhythm follows a natural distribution. A few are less than twenty-four hours long, a few are exactly that long, and many more are longer than twenty-four hours.

Read on to get a full understanding of:

• How long your circadian rhythm is, why there is still some outdated information about the length of your circadian rhythm, and where the length of 25 hours comes from
• How the length of circadian rhythms is normally distributed and what the average length of circadian rhythms is
• How to find out whether the length of your circadian rhythm matches twenty-four hours
• 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.^​1​

The Average Circadian Rhythm

Generalizations About the Length of Our Circadian Rhythm

Let’s start with why there might still be some outdated numbers about the length of your circadian rhythm out there. And to understand where they come from, let me highlight the most important research that led to these. There you will also see where the still popular length of twenty-five hours for our circadian rhythm comes from.

Let’s also not forget that the study of circadian rhythms is still not as advanced as other scientific fields. Even though the first written record about circadian rhythms dates back to 1729^​2​ (it was discovered in the mimosa plant), there was not much progress afterward. For about the next about two-hundred years.^​3​ Especially not for the human circadian rhythm. For yours and mine.

Initially Believed Lengths

Initially Believed Circadian Rhythm Lengths

In the 1950s, researchers started with the first experiment to figure out more about the length of our circadian rhythm. Von Wever and colleagues found a volunteer who would go deep into a cave in the Andes. With lots of food and things to be distracted. But without any daylight or social interactions. Besides a phone to call the researchers when he went to bed and when he woke up.^​4​

The result of this first experiment? Our volunteer had a circadian rhythm of twenty-four hours and fifteen minutes. But as we’ll see, the length of circadian rhythms can vary from person to person. This means that your circadian rhythm might be different from the one from this volunteer.

Later studies reported that our circadian rhythms always show periods that are close to twenty-five hours long.^​5​ But what those studies forgot to eliminate were external factors that influence our circadian rhythms: Those study participants were exposed to light levels that were high enough to confound their circadian rhythms.^​6​

Those studies explain why you can still find information about a twenty-five-hour long circadian rhythm. But now we also know that those are outdated.

So, let’s now have a look at studies that take external factors into account. And that are also high enough in number to show statistical relevance. And, as you’ll see, the length of our circadian rhythms follows – like virtually everything in nature^​7​ – a normal distribution.

Firstly, why are there so many different numbers? Because there is no one length for our circadian rhythm. That also means that I can not give you one simple number for how long your circadian rhythm is. Instead, we can have a look at the normal range of circadian rhythm length. And then have a look at your individual circadian rhythm.

Range of Lengths

The Range of Circadian Rhythm Lengths

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. But one step at a time.

In the early 1960s, Jürgen von Aschoff and Rütger Wever were determined to find out more about the human circadian rhythm.^​4​ So they created an environment that eliminated all possible external factors that could influence our circadian rhythm: they built two apartments in a bunker, located inside a hill. This place was called the “Andechs Bunker.”

• 

• 

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 they find? The circadian rhythms in isolation weren’t exactly twenty-four hours long. For most volunteers, their internal days were slightly longer, just for a small number they were slightly shorter.

Ok, let’s recap here quickly. Your circadian rhythm is not exactly twenty-four-hours long but most likely slightly longer than. Next, we will have a closer look at another study that provided a specific average length for our circadian rhythm.

Average Length

The Average Length of Our Circadian Rhythm

In their monumental 1999 paper, Harvard professor Charles Czeisler and colleagues published the average length of the human circadian rhythm. They studied the sleep-wake cycles of eleven healthy young participants (average age of 23.7 years) and thirteen healthy older participants (average age of 67.4 years) for 29 to 38 days.^​6​

Their result? According to their studies, our circadian rhythm is on average 24.18 hours long. This result was true for both age groups. And it is also in line with the previous “Andechs Bunker” experiments of a circadian rhythm slightly longer than twenty-four hours.

But it is more accurate to give you a range of circadian rhythm lengths that were measured from vast population data. And to also show you how you can measure your circadian rhythm length. So let’s have a look at this now.

Your Circadian Rhythm

The Length of Your Circadian Rhythm

The length of your circadian rhythm is also the length of your internal day. This is expressed through your chronotype. And your chronotype is just the representation of your sleep-wake cycle. 

If the length of your circadian rhythm (and with this the length of your internal day) is longer than twenty-four hours (the length of the external day), then you are a later chronotype. This means that you would naturally go to bed and wake up rather later.

And if the length of your circadian rhythm (the length of your internal day) is shorter than twenty-four hours (the length of the external day), then you are an earlier chronotype. This means that you would go to bed and wake up rather earlier. 

In short, through your chronotype (read: through your sleep-wake cycle) you can find out the length of your circadian rhythm.

In their 2007 landmark paper, leading chronobiologist Till Roenneberg and Martha Merrow studied the sleep patterns of more than 55,000 people that they collected through their Munich ChronoType Questionnaire. A database that has now grown to over 300,000.^​8​

To make sense of their data, let me tell you a little more about your chronotype: Your chronotype is the expression of the length of your circadian rhythm. And this expression also depends on social as well as environmental factors. So your chronotype is basically, how you express your circadian rhythm in real-life conditions.

And you can calculate your chronotype through finding out your midsleep point (that is the middle between when you fall asleep and when you wake up) on the free days. When you can self-select when you want to go to sleep and wake up. Without social pressures to stay up longer at night or an alarm clock in the morning that would reduce your sleep time.

This is how you can calculate your chronotype (midsleep point of your free days):

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.

Now, with your midsleep point (as a reference point for your chronotype) in mind, have a look at the following two graphs:

• 

• 

Let’s compare these two graphs now to find out more about what they can tell us about the length of circadian rhythms in real life.

Have a look at the left graph now. The black dots represent the sleep duration on workdays. And the white dots the sleep duration on free days. If internal and external days are not aligned then those differ. 

There are three basic outcomes from this data:

1. You have a midsleep point (chronotype) later than 3 am: You get not enough sleep on your workdays and have to catch up through more sleep on your free days. This is the case if your internal day is longer than your external day. And this means that your 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 internal day is as long as your external day. And this means that your 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 not enough (think: social pressures) on your free days. This is the case if your internal day is shorter than your external day. And this means that your 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”

The important point about your circadian rhythm here is that it is your circadian rhythm in real life. With all the social and environmental factors that influence it. In isolation and without those, your free-running circadian rhythm might be different. But, then, you neither live in isolation nor without social and environmental factors. So let’s continue.

Have a look at the right graph now. There you can see the relative distribution of chronotypes based on their midsleep points. And keep in mind that a midsleep point of 3 am is our reference point here for a circadian rhythm that is twenty-four hours long.

Let’s have a look at the percentage of the population and their circadian rhythm length:

1. Less than five percent have circadian rhythms that are shorter than twenty-four hours
2. About six percent circadian rhythms of exactly twenty-four hours
3. Close to ninety percent have circadian rhythms longer than twenty-four hours

Your Implications

What Does the Length of Your Circadian Rhythm Mean for You

Our circadian rhythm is not exactly twenty-four hours long because it never had to be. There were no evolutionary pressures for this. Instead, we developed a rhythm that is only about twenty-four hours long and that has to be influenced by our environment to match our external day length. 

How does this work? In short, The light signals your brain receives align your internal circadian rhythm to your external day length. At least that is how it works if you get plenty of natural light during the day. 

If your natural light exposure during the day, however, is too low and/ or your artificial light exposure in the evening and at night is too high, then your body can’t align your circadian rhythm to your external day. Now, if your circadian rhythm normally is longer than twenty-four hours, and your body can’t fully align it, then you will have issues falling asleep in the evenings and waking up the next mornings.

Now, what concrete steps can you take to align your circadian rhythm to your external day? So that it ends up being twenty-four hours long? I’m glad you asked! Check out this post about “How to Get Your Circadian Rhythm Back on Track.”

Personal Experiences

My Personal Experiences

I know that my circadian rhythm is slightly longer than twenty-four hours. That means that I have to align it during the day through plenty of natural light. This light during the day speeds up my circadian rhythm until it matches the external twenty-four hour day.

What has especially helped me in this process are outside activities early in the morning. Like jogging or walking. And I can feel a big difference when my body is ready to sleep in the evening. And how good my sleep quality is.

Key Takeaways

Key Takeaways

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

1. Your circadian rhythm is most likely longer than twenty-four hours.
2. You can find it out by your sleep-wake behavior during the week and on weekends. At least the one with all your social and environmental influences. For your free-running circadian rhythm, you’d have to eliminate all those influences 
3. You need to align your circadian rhythm to the external twenty-four hour day. This process is called entrainment and happens through your natural light exposure.

And now back to you: Is your circadian rhythm aligned to your external twenty-four-hour day length? Is there a difference for you during the week and on weekends?

Stay fit,

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

References

References

1. 1.

   HALBERG F. [Physiologic 24-hour periodicity; general and procedural considerations with reference to the adrenal cycle]. Int Z Vitaminforsch Beih. 1959;10:225-296. https://www.ncbi.nlm.nih.gov/pubmed/14398945.
2. 2.

   Zordan M, Costa R, Macino G, Fukuhara C, Tosini G. CIRCADIAN CLOCKS: WHAT MAKES THEM TICK? Chronobiology International. January 2000:433-451. doi:10.1081/cbi-100101056
3. 3.

   Roenneberg T. Internal Time. Harvard University Press; 2012.
4. 4.

   Von Aschoff J, Wever R. Spontanperiodik des Menschen bei Ausschluß aller Zeitgeber. Naturwissenschaften. August 1962:337-342. doi:10.1007/bf01185109
5. 5.

   Wever R. Characteristics of circadian rhythms in human functions. J Neural Transm Suppl. 1986;21:323-373. https://www.ncbi.nlm.nih.gov/pubmed/3462338.
6. 6.

   Czeisler CA. Stability, Precision, and Near-24-Hour Period of the Human Circadian Pacemaker. Science. June 1999:2177-2181. doi:10.1126/science.284.5423.2177
7. 7.

   FRANK SA. The common patterns of nature. Journal of Evolutionary Biology. July 2009:1563-1585. doi:10.1111/j.1420-9101.2009.01775.x
8. 8.

   Roenneberg T, Merrow M. Entrainment of the Human Circadian Clock. Cold Spring Harbor Symposia on Quantitative Biology. January 2007:293-299. doi:10.1101/sqb.2007.72.043