You’ve certainly noticed that some things change as you age. Like that you can’t stay up all night long anymore. And maybe also that you don’t mind waking up earlier in the morning. But did you know that it’s not just your sleeping pattern that changes but also everything else controlled by your circadian rhythm? Which means the timing of virtually every body function.

How does your circadian rhythm change as you age? You start with an early circadian rhythm in your childhood that gets later and later until it reaches its peak when you are around 20 years old. From this turning point on, your circadian rhythm gets progressively earlier again. Unfortunately, your circadian rhythm also gets weaker as you age.

Read on to get a full understanding of:

• How your circadian rhythm changes over your lifetime and why your sleep-wake cycle directly reveals those changes
• What the circadian rhythm graph looks like for both genders, starting at age ten up to age eighty
• What the main differences are between circadian rhythms at a young age and old age
• Why your circadian rhythm changes and what the role of light and your central pacemaker is
• My personal experiences and your key takeaways

Let’s start with a quick overview of how your circadian rhythm controls the daily timing of virtually all your body functions – including your sleep. And how we can use your sleep patterns to gain insights into your circadian rhythm and how it changes over your lifetime.

The How

How Your Circadian Rhythm Changes Over Your Lifetime

To start with how your circadian rhythm changes over time, I need to give you a quick introduction to your circadian rhythm and how it can be measured. And then we’ll have a look at the circadian rhythm changes across your lifespan. Promised. (Otherwise, just feel free to skip to the next section.)

Your circadian rhythm is like a daily schedule that your body follows to optimize all its functions. This is important because:^​1​

• Your body can’t perform all its functions once.
• Some functions only make sense during specific times of the day.

What body functions follow your circadian rhythm? Virtually every function of your body, including your hormones, your neurotransmitters, and your core body temperature.

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

And how do you feel these timely optimizations in your daily life? Those impact things like your peak physical and cognitive performance during the day or when and for how long you are able to sleep at night.

Let’s have a look at this last part about sleep now.

The connection between sleep and your circadian rhythm has been established for many decades as the “two-process model of sleep.” This model, at its core, explains your ability to sleep as a combination of how much sleep debt you have built up and whether it’s the right to sleep time based on your circadian rhythm (aka whether your circadian rhythm also allows you to sleep).^​2–4​

In short, your circadian rhythm has a dominant impact on when and for how long you are able to sleep. And your sleep patterns (also called your chronotype) consequently reveal a great deal of general information about your chronotype.

The foremost way to study circadian rhythms in real life is through studying self-reported sleep timings:^​1,5​

• Sleep patterns highly correlate with the circadian (daily) patterns of other hormonal and biochemical factors, like core body temperature or cortisol and melatonin levels.
• People are remarkably precise in estimating their usual sleep times.

This is why chronobiologists (those researchers that study circadian rhythms) have collected large sample sizes of sleep-patterns. And this data reveals how circadian rhythms change across our lifespan.

Changes With Age

How Does Your Circadian Rhythm Change Across Your Lifespan

Let me introduce you to the largest study of circadian rhythms in real life: The Munich ChronoType Questionnaire. To date, more than 300,000 entries across virtually all age groups reveal data about participants’ circadian rhythms through their sleep-wake cycles (aka their chronotypes).^​6​

And as we’ve seen above, the circadian impact on your sleep is so dominant that you can have a look at your sleep-wake cycles (your chronotype) to find out more about your underlying circadian rhythm. And also how it both compares to the population and changes over time.

So, how would you calculate your sleep-wake cycles? Through calculating your midsleep point. Which is just as it sounds: the point in the middle of your sleep. And it elegantly includes the most important data points from when you fall asleep and wake up to how long you sleep:

• Write down the time when you fall asleep.
• Write down the time when you wake up.
• Take the middle of those times.

But only use “free” days, when you can self-select at what time you fall asleep and wake up. And when you have enough midsleep points, simply take their average.

Do you want to get to know the full story behind it? Check out this post: “What Are Chronotypes and How to Find Out Yours”

Here’s one example: Let’s say you fall asleep at midnight and then wake up at 8 am. Then your midsleep point would sit just in the middle at 4 am. And if you fall asleep at 1 am and wake up at 10 am? Then your midsleep point would be 4:30 am.

What does that mean for your circadian rhythm? It gives you a direct insight into it!

• Your circadian rhythm is a dominant factor for your midsleep point.^​2–4​
• And your midsleep highly correlates with many aspects of your circadian rhythm.^​1,5​

Ok, back to your midsleep points. Did you calculate them? Great!

Let’s compare them with population data now. This data comes directly from the largest studies about chronotypes and from leading chronobiologist Till Roenneberg.^​1,7​

• 

What you can see in this graph are the average chronotypes (represented through their midsleep points) by age. And the higher the line, the later the chronotype is (their midsleep points are).

But what about babies you might ask? Well, they are initially governed by a rhythm that is much shorter than twenty-four hours. And only after a few months, they gradually develop a daily (fully circadian) rhythm.^​7​

Here are the most important findings from the Munich ChronoType Questionnaire studies:^{​1,5,8,9​}

• You start as an early chronotype in your childhood.
• But then you progressively become a later chronotype until around the age of 19.5 if you are female or 21 if you are male. (Btw, this one-and-a-half-year sex difference is typical for many aspects of human development.)
• From this turning point onwards, you become an earlier and earlier chronotype for the rest of your life. And because men continue to delay their chronotype for that bit longer, they also tend to be later chronotypes than women for most of their adulthood.
• This gender difference in your chronotype becomes smaller with age and vanishes around the age of 52 (which coincides with the average age of menopause).
• When you are over 60 years of age, you on average become even earlier chronotypes than you were in your childhood.

Now, do you remember how your circadian rhythm impacts your chronotype and how your chronotype highly correlates with many markers of your circadian rhythm? Why? Because your circadian rhythm controls the timing of all your body functions – including sleep (especially through changes in your core body temperature and the hormones melatonin and cortisol). You can view your sleep-wake cycle (your chronotype) as the most direct expression of your circadian rhythm.

Now, have a look at the above list again knowing that these changes over time also reflect changes in your circadian rhythm. 

Now, what does that mean for your daily life?

• You’ve seen that your circadian rhythm is like the daily schedule that your body uses to optimize the timing of its functions. And because this schedule changes its timings as you age, also your body functions, including your sleep, change over time.
• In a way, when you see the change in your circadian rhythm, you also see the changes in your hormones. And how they, already at the age of around twenty years, progress throughout your life. Their daily timings become earlier and, as we’ll see in the next part, their amplitudes become more and more dampened.

Have you noticed any of those changes over time during your life so far? Maybe not explicitly. Or maybe you haven’t attributed those changes to changes in your circadian rhythm yet. But now that you know, it’s much easier to not just link any changes that have happened in the past but also those that will come in your future. Or to simply better understand some behaviors from people of different ages.

There are two more things to keep in mind when looking at this data:

1. What does it mean if your midsleep point doesn’t match the one from the graph above? Within each age group, chronotypes follow a wide (slightly skewed towards the later end) normal distribution. But the graph above only shows the single average point. So, your chronotype doesn’t necessarily match the average for your age group and sex. But the general trend of your chronotype follows a similar pattern over your lifetime.^​1​
2. Midsleep points are not only different for different age groups but also vastly differ depending on whether they are measured during the week or on the weekend. Midsleep times are generally later on free days when people can self select their sleeping times. But this gap is by far the widest for adolescents (those at the peak of the curve) who delay their sleep by almost three hours on weekends. Why? Because they need to catch up on the sleep debt that they accumulated during the week – when they are forced to wake up too early for their chronotype.^​5,8​

Here, you’ve seen the big picture of how your circadian rhythm (through the expression of your chronotype aka midsleep points) changes throughout your lifetime.

Let’s now have a look at those changes in more detail. Thanks to studies that compared the circadian rhythms of younger adults with those of older adults.

Young Age vs Old Age

What Is the Difference Between Your Circadian Rhythm at Young Age and Your Circadian Rhythm at Old Age

In the previous part, you’ve seen the most noticeable result of a change in your circadian rhythm: the change in your natural sleeping preferences aka your chronotype.

In this part, we’ll have a look into more detail at the underlying changes within your body that come along when your circadian rhythm changes with age. And then at the implications for you.

Canadian researchers Suzanne Hood and Shimon Amir visualized those changes best in their paper “The aging clock: circadian rhythms and later life.” So let’s use this visualization as our starting point:^​10​

• 

What is the first thing that you notice when you compare the circadian rhythms of older adults (red lines) with those of younger adults (blue lines)? Yes, the red lines look more flattened. Meaning that the circadian rhythms of older adults are weaker. Or, in more scientific terms, the amplitudes of their circadian rhythms are dampened.

And now to the second thing that you notice when you compare their respective circadian rhythms? Yes, the peaks (both high and low) of their lines appear earlier for older adults. Meaning that the circadian rhythms of older adults are earlier. Or, in more scientific terms, their circadian rhythms are phase advanced.

Now, I’d like you to focus on two of those circadian rhythms for now: “SCN firing” and “temperature”. Why? Because they are central to explain all other circadian rhythm changes throughout your body:

• Your SCN (in full: suprachiasmatic nucleus) is your master circadian pacemaker. It is a tiny region in your brain that is responsible for centrally controlling your circadian rhythm.^​11​
• Your SCN aligns your circadian rhythm throughout your body through changes in your core body temperature.^​12​

How does that translate to your daily life? Well, if your central circadian rhythm (your SCN) is weaker and your core body temperature has its ups and lows earlier, then it is not surprising that everything else is also either weaker or earlier or both.

Just as in the “waking activity” graph and in our previous chronotype study. The older you get, the more you change toward a morning person. Not just with your sleep behavior, but also with everything else.

Let’s put it in the words of Suzanne Hood and Shimon Amir, authors of our study:^​10​

“The preference for morningness in older adulthood is expressed in other aspects of behavior, such as cognitive skill performance”

Suzanne Hood and Shimon Amir

This is also in line with previous studies from Till Roenneberg, who showed that both the concentration and timing of the release of many hormones are age-dependent.^​1​

Remember that your daily life is heavily influenced by your hormone levels. And the daily cycle of your hormone is driven by your circadian rhythm. Just like the combination of a high melatonin level and a low cortisol level, together with a low core body temperature helps you to sleep, other hormones play crucial roles in other parts of your life.^​13,14​

• A shift toward an earlier sleep-cycle (chronotype) is the most consistently observed age-related circadian change (not just for us humans, but in many species).^{​1,10,13​}
• Your performance (as measured through both memory and reaction time) has a changed time-of-day pattern as you age – with peak performances occurring earlier and at reduced accuracy and speed.^​10​
• Your body releases your growth hormone about one hour earlier in your old age vs your young age.^​1,8​
• Also, your rhythmic gene expression in your brain is altered as you age and contributes to changes in cognition and mood later in life.^​13​

In short, the biggest change with age is that your circadian rhythm becomes earlier. But another big change that seems to come with age is a weakened circadian rhythm or even a complete or partial loss in rhythmicity.

So, let’s have a look at the main reasons behind this weakened or disrupted circadian rhythm at age and what you could do to keep your circadian rhythm strong.

The Why

Why Your Circadian Rhythm Changes Over Your Lifetime

To better understand why your circadian rhythm gets earlier and weaker (or even disrupted) with age, let’s start with a basic summary of how it works:^{​11,12,14​}

• You’ve seen that your central circadian rhythm is controlled by a tiny part of your brain called the SCN (suprachiasmatic nucleus).
• Your SCN depends on light information from your environment to align your time with your external day.
• And your SCN then orchestrates all the other rhythmic activity in your body through changes in your core body temperature.

You can read the full story in “How Does Your Circadian Rhythm Work: All You Need to Know”

Both your light exposure and what your SCN does with this are central to your circadian rhythm. Those are the two main points where your SCN – the tiny part in your brain that controls your circadian rhythm – could change with age:

1. Through the light information that your SCN receives from your environment (about the time of day).
2. And what your SCN does with this light information.

And, you guessed correctly, both of these change as you age. And it is because of these changes that also your circadian rhythm changes. So let’s dive a little deeper into each.

First, the light information that your SCN (your central circadian rhythm) receives:

• Unfortunately, your eyes are getting worse as you age. To be more specific, your lens progressively yellows and thickens. That means that your lens reduces your sensitivity to light as you age. Reducing the strength of your most important environmental cue to align your circadian rhythm.^​10​
• But then, also your SCN gets worse at receiving light information from your eyes. The aging SCN suffers from a structural reorganization of its light-receiving components, ultimately reducing its light sensitivity. Unfortunately, this further impairs its function in setting and maintaining a stable circadian rhythm.^​15​
• Finally, most elderly people are simply not exposed to enough natural light anymore. In many cases, they hardly ever get the chance to go outside. And the television is often their main light source.^​16,17​

Let’s go to a rather extreme environment of old age and have a quick look at nursing homes now. Why? Because this highlights the importance of your light exposure.

Eus van Someren, now department head at the Netherlands Institute for Neuroscience, and his team recorded the activity levels of the elderly living in nursing homes. And found out that their nighttime sleep was disrupted by activity and, conversely, how much their daytime activity was disrupted by naps.^​17​

They also found that those elderly were not exposed to enough natural light anymore. So they installed bright light sources in the common rooms of these homes. The result? The patients’ circadian rhythms became stronger again and both their nightly and daily disruptions became much less frequent.

Second, what your SCN (your central circadian rhythm) does with that light information:

• Firstly, it is still largely unknown why the circadian rhythm gets earlier as we age. Some studies show a shortened circadian rhythm in animals as they age (which would explain an earlier timing). But those could not be replicated with us humans.^​18,19​
• The SCN has weaker circadian regulation with reduced amplitudes in circadian rhythmicity – especially in your core body temperature (which is key in aligning your circadian rhythms throughout your body).^​20​
• Your SCN doesn’t as effectively orchestrate your rhythmic activity throughout your body.^​10​
• And you need longer to adapt to new time zones as you age – especially if you need to phase advance (aka fly east).^​10​
• The aging SCN also suffers from neuronal degeneration that could particularly drive circadian misalignment throughout your body.^​20​
• As a result of those age-related changes to your SCN, your circadian rhythm, in general, becomes less robust in old age.^​1,9​

Let me tell you about arguably the first human experiment to find out more about circadian rhythms in the absence of environmental cues. And how this one shows the differences to adapt to a new circadian rhythm between at different ages.

In 1938, Daniel 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 to study their circadian rhythms.^​21​

They tried to adapt to a self-imposed twenty-eight hour day in the absence of any environmental cues (such as light) that could give away any real time-of-day information. The result? The twenty-year-old Richardson was able to adjust his circadian rhythm to the new cycle after just about a week. While the forty-three-year-old Kleitman couldn’t adapt his circadian rhythm even until the end of this experiment.

Now, what are the implications of all this for you? First, don’t go into a cave and try to adapt to a new circadian rhythm in the absence of any environmental cues.

Instead, use the environmental cues that help to align your circadian rhythm to your time of day to your advantage. And as you’ve seen, you need to pay special attention to the by far most powerful environmental cue: your daily light exposure.

Here are the top tips to increase your light exposure:

1. Spend at least two hours outdoors during the first half of every day
2. Stay as close to windows as possible if you are indoors
3. Take artificial light showers during the day if needed
4. Don’t wear sunglasses (if you don’t spend the whole day outside)

I can strongly recommend you to get the full overview. Especially since most circadian rhythms are already weakened and/ or disrupted at an early age. Check out this post: “How to Get Your Circadian Rhythm Back on Track: The Ultimate Guide”

Personal Experiences

My Personal Experiences

When I think about how my circadian rhythm has changed so far, my sleep pattern is what directly comes to my mind. And while I can’t remember having had any issues falling asleep and waking up early as a kid, things definitely changed in my later teenage years.

My first memories of partying are also connected with staying up all night long. And with then being able to sleep in all day long. But waking up early to go to school? That always felt like torture – as if my sleep had been interrupted in the middle of the night (and as we know, from the perspective of my body, that’s literally what happened).

But now? Not so much anymore.

I’m happily waking up quite early in the morning (also without an alarm clock). My sleep drive in the evening seems to only get stronger earlier. And while occasional long nights still do happen, I’m simply not able to sleep in anymore.

And as we’ve seen, those changes that I’m experiencing (and will most definitely also be true for you) are simply a combination of how your sleep works and how your circadian rhythm changes.

Do you remember (from the two-process model of sleep) that one dominant factor that determines when and for how long we are able to sleep is our circadian rhythm?

• The two dominant factors for my sleep are the sleep pressure that I build up being awake and whether my body is ready to sleep based on my circadian rhythm.
• Well, in my (late) teenage years, my circadian rhythm was late. That moved my whole sleep cycle later too. It felt more natural to both stay up late and sleep in long. But it definitely didn’t feel natural to wake up early.
• Now that my circadian rhythm has become earlier again, also my whole sleep cycle is moved earlier. And I feel more in tune with my body waking up and going to bed rather early.

Now, those changes are just a natural part of our life. But what we can do is to strengthen our circadian rhythm through the right light exposure.

For example, I love to get in natural sunlight directly at the beginning of the day by way of a short walk or easy run outside. And I also try to work either outside (if possible), next to a window, or otherwise with those artificial light showers. One on each side of my laptop.

Key Takeaways

Key Takeaways

There are three key takeaways that I want to share with you that all help you to use environmental cues to your advantage to entrain and ultimately strengthen your circadian rhythm:

1. The circadian impact on your sleep is so dominant that you can have a look at your sleep-wake pattern (your chronotype) to find out more about your underlying circadian rhythm
   • You start as an early chronotype in your childhood.
   • You start as an early chronotype in your childhood.
   • But then you progressively become a later chronotype until around the age of 19.5 if you are female or 21 if you are male.
   • From this turning point onwards, you become an earlier and earlier chronotype for the rest of your life. This gender difference in your chronotype becomes smaller with age and vanishes around the age of 52.
   • When you are over 60 years of age, you on average become even earlier chronotypes than you were in your childhood.
2. Your circadian rhythm when you are an older adult is both earlier (phase advanced) and weaker (with dampened amplitudes) as compared to your circadian rhythm when you are a younger adult.
3. Your central circadian rhythm is governed by a tiny part of your brain called the SCN (suprachiasmatic nucleus). And this part becomes less robust over time, which weakens your circadian rhythm.
   • As you age, your SNC receives less light input because both your lens and the structure of your SCN become less light-sensitive with age and because you generally are not exposed to enough natural light anymore.
   • At the same time, your SCN gets weaker at regulating and orchestrating your circadian rhythm throughout your body and also needs longer to adapt to new time zones.

And now back to you: Have you already recognized how your circadian rhythm has changed over time? And have you already noticed the positive impact you can have with optimizing your daily light environment?

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.

   Roenneberg T, Kuehnle T, Juda M, et al. Epidemiology of the human circadian clock. Sleep Med Rev. 2007;11(6):429-438. doi:10.1016/j.smrv.2007.07.005
2. 2.

   Borbély A. A two process model of sleep regulation. Hum Neurobiol. 1982;1(3):195-204. https://www.ncbi.nlm.nih.gov/pubmed/7185792
3. 3.

   Achermann P. The two-process model of sleep regulation revisited. Aviat Space Environ Med. 2004;75(3 Suppl):A37-43. https://www.ncbi.nlm.nih.gov/pubmed/15018264
4. 4.

   Borbély AA, Daan S, Wirz-Justice A, Deboer T. The two-process model of sleep regulation: a reappraisal. J Sleep Res. Published online January 14, 2016:131-143. doi:10.1111/jsr.12371
5. 5.

   Roenneberg T, Wirz-Justice A, Merrow M. Life between Clocks: Daily Temporal Patterns of Human Chronotypes. J Biol Rhythms. Published online February 2003:80-90. doi:10.1177/0748730402239679
6. 6.

   MCTQ -. Munich ChronoType Questionnaire. The WeP. Published 2020. https://www.thewep.org/documentations/mctq/item/mctq
7. 7.

   Roenneberg T. Internal Time. Harvard University Press; 2012.
8. 8.

   Roenneberg T, Kuehnle T, Pramstaller PP, et al. A marker for the end of adolescence. Current Biology. Published online December 2004:R1038-R1039. doi:10.1016/j.cub.2004.11.039
9. 9.

   Roenneberg T, Keller LK, Fischer D, Matera JL, Vetter C, Winnebeck EC. Human Activity and Rest In Situ. In: Methods in Enzymology. Elsevier; 2015:257-283. doi:10.1016/bs.mie.2014.11.028
10. 10.

    Hood S, Amir S. The aging clock: circadian rhythms and later life. Journal of Clinical Investigation. Published online February 1, 2017:437-446. doi:10.1172/jci90328
11. 11.

    Weaver DR. The Suprachiasmatic Nucleus: A 25-Year Retrospective. J Biol Rhythms. Published online April 1998:100-112. doi:10.1177/074873098128999952
12. 12.

    Buhr ED, Yoo S-H, Takahashi JS. Temperature as a Universal Resetting Cue for Mammalian Circadian Oscillators. Science. Published online October 14, 2010:379-385. doi:10.1126/science.1195262
13. 13.

    Chen C-Y, Logan RW, Ma T, et al. Effects of aging on circadian patterns of gene expression in the human prefrontal cortex. Proc Natl Acad Sci USA. Published online December 22, 2015:206-211. doi:10.1073/pnas.1508249112
14. 14.

    Roenneberg T, Merrow M. Entrainment of the Human Circadian Clock. Cold Spring Harbor Symposia on Quantitative Biology. Published online January 2007:293-299. doi:10.1101/sqb.2007.72.043
15. 15.

    Biello SM, Bonsall DR, Atkinson LA, Molyneux PC, Harrington ME, Lall GS. Alterations in glutamatergic signaling contribute to the decline of circadian photoentrainment in aged mice. Neurobiology of Aging. Published online June 2018:75-84. doi:10.1016/j.neurobiolaging.2018.02.013
16. 16.

    Riemersma-van der Lek RF. Effect of Bright Light and Melatonin on Cognitive and Noncognitive Function in Elderly Residents of Group Care Facilities. JAMA. Published online June 11, 2008:2642. doi:10.1001/jama.299.22.2642
17. 17.

    Van Someren EJW, Kessler A, Mirmiran M, Swaab DF. Indirect bright light improves circadian rest-activity rhythm disturbances in demented patients. Biological Psychiatry. Published online May 1997:955-963. doi:10.1016/s0006-3223(97)89928-3
18. 18.

    Duffy JF, Zitting K-M, Chinoy ED. Aging and Circadian Rhythms. Sleep Medicine Clinics. Published online December 2015:423-434. doi:10.1016/j.jsmc.2015.08.002
19. 19.

    Duffy JF, Czeisler CA. Age-related change in the relationship between circadian period, circadian phase, and diurnal preference in humans. Neuroscience Letters. Published online February 2002:117-120. doi:10.1016/s0304-3940(01)02427-2
20. 20.

    Schmidt C, Peigneux P, Cajochen C. Age-Related Changes in Sleep and Circadian Rhythms: Impact on Cognitive Performance and Underlying Neuroanatomical Networks. Front Neur. Published online 2012. doi:10.3389/fneur.2012.00118
21. 21.

    Kleitman N. Sleep and Wakefulness as Alternating Phases in the Cycle of Existence. University of Chicago Press; 1939.