To know how your circadian rhythm works enables you to take a great step towards living aligned with it. In this article, I will outline how light is the main external factor that impacts your circadian rhythm. How your body interprets the light signals it receives from your eyes. And how this centrally affects your circadian rhythm. Let’s get started with a quick overview:
How does your circadian rhythm work? Your circadian rhythm is centrally controlled by a part of your brain called the SCN. The SCN gets your time of day information through light receptors in your eyes that are especially sensitive to blue light. Through this light information, it adjusts your circadian rhythm to your external day.
Read on to get a full understanding of:
- Why light is the key component to set your circadian rhythm
- What the special role of blue light in this process is
- How one tiny part of your brain (the SCN) sets your circadian rhythm
- How you adapt your circadian rhythm to external light information (entrainment)
- When and why we stopped living by sun time
- A real-world case study from Germany about the impact of sun time and light pollution
- My personal experiences and your key takeaways
You might have guessed already: light plays a key role in setting your circadian rhythm. Especially blue light rays. That’s why we start with light and how it reaches your brain. And then we have a look at how your brain uses this light information to set your circadian rhythm. But first things first. Let’s honor that discoveries in these areas were awarded a Nobel Prize in 2017.
Light Sets Your Circadian Rhythm
How Your Body Uses Light to Set Your Circadian Rhythm
The Nobel Prize in Physiology or Medicine 2017 was awarded jointly to Jeffrey C. Hall, Michael Rosbash and Michael W. Young “for their discoveries of molecular mechanisms controlling the circadian rhythm.”1
“Life on Earth is adapted to the rotation of our planet. For many years we have known that living organisms, including humans, have an internal, biological clock that helps them anticipate and adapt to the regular rhythm of the day. […] Their discoveries explain how plants, animals and humans adapt their biological rhythm so that it is synchronized with the Earth’s revolutions.“The Nobel Assembly at Karolinska Institutet
What I am going to share with you is the way your body adapts your biological rhythm – your circadian rhythm. One step at a time. Let’s start from the beginning. Let’s start with the different time systems that you interact with.
Disclaimer: You have one central (master) circadian rhythm that is by far the most powerful and important one for you. And it aligns and orchestrates all your other circadian rhythms. This is why I only speak about your central (master) circadian rhythm in this post. And I refer to it simply as your circadian rhythm. If you want to find out more about your other circadian rhythms, I can recommend you this post: “How Many Circadian Rhythms Do We Have”
Why Your Body Still Works Based on Sun Time
There are three different time systems that your body interacts with every day: sun time, social time, and your internal time:2
- Sun time is the time of nature. Your body receives visual feedback through the position of the sun in the sky and the respective sunray pattern.
- Internal time is the time your body thinks it is. For this, you have an internal biological clock. Through your biological clock, your body maps its internal time. And through its internal time, your body develops your daily rhythm – your circadian rhythm.
- Social time is the time you live by based on your environment and social commitments. This could be anything from work or study schedule to time commitments with friends and family.
Now to the big questions: Which of those times has the biggest impact on your lifestyle? Yes, your social time. And which has the biggest impact on your circadian rhythm? Ok, that was a trick question. It is the alignment of all those three.
Sneak peek: Your body aligns your internal time through sun time. At least if your sun exposure is great enough. And your circadian rhythm is based on your internal time. Your circadian rhythm regulates all your functions in a way that it matches what should happen based on the specific time of day it thinks it is. And if your social time (remember, all that social fun plus those commitments?) is aligned with your circadian rhythm, then you set yourself up to become your best self.
But why is your body so strongly connected to sun time? The quick summary is because the sun behaves cyclically and predictably. So we had an evolutionary advantage to adapt our bodies to sun time, in a way that our bodies also behave cyclically and predictably.
For a full overview, check out this post about “What Is the Circadian Rhythm and Why Do We Have One.”
And, unlike the two states of day and night, your circadian rhythm is more than just the difference between being awake or sleeping. All your bodily functions go through a constant change. Wakefulness and sleep are only the two most obvious states.
Ok, the first part of how your circadian rhythm works has to do with how it is influenced by light. So let’s have a close look at the pathway from light through your eyes and then to your internal clock where it impacts your circadian rhythm. Shall we?
How Your Brain Receives Light Signals About the Time of Day
Light is the main signal that your body (and virtually every other organism) uses to uses to align your internal clock. And through your internal clock, you establish your circadian rhythm.
Let’s track the way of light, to see how exactly your body uses it to set your internal time and circadian rhythm:3–5
- Light comes from the outside and enters your body through your eyes. To be more precise, it enters your body through the lenses of your eyes.
- From your lenses, light is then projected onto the back of your eyeballs.
- The back of your eyeball is covered with a layer of cells that has millions of light receptors. This layer is called the retina.
- The retina has receptors that translate different light information for the brain:
- You might have heard about the two kinds of light receptors in the retina, the rods and cones. Rods help you see grayscale and contrast (especially in dim light conditions). Cones help you see color (when enough light is available)
- But, as recent as in 1998, scientists discovered another light receptor: melanopsin. Melanopsin is a protein on about five percent of your retina cells. It does not help you see, but it collects information about the light brightness. Melanopsin helps you sense the sun time of day. And you might have guessed correctly, melanopsin is the light receptor that influences your circadian rhythm.
- Your retina is connected to your brain through the optic nerve. Accordingly, it sends all light information to the brain via the optic nerve.
To recap, light enters your eyes through your lenses, falls onto your retina, the light receptors there (called melanopsin) translate this light information, and then send this information to the brain via the optic nerve. Are you still with me? Good!
In this process, melanopsin is the gatekeeper to tell your brain which sun time of day it is. So let’s dig a little deeper into how melanopsin works together with your brain. This will help us understand how different kinds of light impact your circadian rhythm.
How Blue Light Impacts Your Circadian Rhythm
Melanopsin helps your brain sense the time of day through information about light brightness. But it does not sense all colors of the light spectrum (the light waves) equally. Melanopsin is most sensitive to blue light waves. Red light waves, on the other side of the spectrum, trigger it much less.6,7
This means that you need a lot of red light intensity to trigger your melanopsin to register any light presence. Spending time at the campfire, for example, will not trigger your melanopsin much. But, since melanopsin is most sensitive to blue light rays, you need relatively less blue light intensity to trigger your melanopsin. And many objects of your daily life emit blue light rays. Any screens from your phone to your computer or TV or modern LED light bulbs.
Here is the important part for your circadian rhythm: When your melanopsin is activated by registering a blue light, it sends a signal to your brain that any light is present, and your brain responds by adjusting your internal time toward daytime – regardless of what time it really is.
What does that mean for your circadian rhythm? Let’s follow the process of blue light you are looking at:
- Blue light rays trigger your melanopsin light receptors in the back of your eye (on your retina).
- Your retina sends your brain the information that light is present.
- For your brain, the presence of any light means that it must be daytime. It adjusts your internal time accordingly – independent of what time of day it really is.
- Your adjusted internal time then impacts your circadian rhythm.
In short, blue light activates your melanopsin light receptors. And this light information tells your brain when it is morning and when it is night. It adjusts your internal time and with this also your circadian rhythm.
But here comes the tricky part. Your everyday light exposure most likely isn’t intensive enough for your melanopsin to tell your brain that it is daytime. And too intensive for your brain to think that it is nighttime.
Light intensity as perceived by your eye is measured in lux. A normally lit room has a light intensity of about 100 lux. A cloudy, rather dark sky starts at about 1,000 lux and a bright, sunny sky can go above 150,000 lux. You need those outdoor light intensities to make your circadian rhythm work well. Indoor light intensities just aren’t enough. They weaken your circadian rhythm.2,5
Now to the second part of this story. You don’t need a high blue light intensity to trigger your melanopsin enough to make your brain believe that it is not nighttime. Remember those 100 lux in your room? Well, a typical night sky has far less than 1 lux of light intensity. And your blue-light-emitting screens only make matters worse. They are intensive enough to interfere with your circadian rhythm. In this way, they also weaken your circadian rhythm.
In short, our modern lifestyle interferes with melanopsin on a daily basis. You are most likely not getting enough (blue) light during the day and too much during the evening and night. This weakens your circadian rhythm. It prevents your body from getting ready for both day and night.
And the easy solution for that? Spend more time outdoors during the day and less time in front of bright blue-light-emitting screens during the evening and night. Or at least block those blue lights either through changing your screen settings to night mode or by wearing blue-light-filtering glasses. Who would have thought that it could be that simple?
Get the full insights in this post about “How to Get Your Circadian Rhythm Back on Track.”
So far I have mentioned the importance of light, especially blue light, for your circadian rhythm. And that your brain sets your internal time and your circadian rhythm with this light information. But it is actually only one tiny part of your brain that is in charge of that.
Light and Your Master Clock
How Your Internal Master Clock (Your SCN) Is Affected by Light
Remember that I mentioned the optic nerve beforehand as the way from your retina into your brain? Well, there is one optic nerve going from your left eye into your brain and one from your right eye. And both these connect to both the left part of your brain and the right part of your brain.
That means that parts of the optic nerve of your left eye have to cross over to the right half of your brain. And parts of the optic nerve of your right eye have to cross over to the left half of your brain.
Why am I telling you this? Because the part of your brain that is in charge of your internal clock and, hence for your circadian rhythm, is located just above this optic nerve crossing. It is located just behind the bridge of your nose.
By the way, this optic nerve crossing is called the optic chiasm. And the part of your brain that centrally controls your circadian rhythm is called the suprachiasmatic nucleus, commonly known as the SCN. Supra means above, so it is the above-the-(optic)chiasm nucleus. Doesn’t sound that sophisticated anymore that name, right?
Ok, enough of those terms for now. The important thing to keep in mind is that it is your SCN that centrally controls your circadian rhythm. And it does so with the help of your melanopsin receptors.
Here are some quick facts about your SCN:5,8,9
- Your SCN is made up of about 20,000 cells, which is a tiny fraction of your brain. Your brain has a total of about 100 billion neurons. That is five million times the size of your SCN.
- Your SCN uses light information from the outside world to adjust your internal time. For this, it has a direct connection to your melanopsin receptors. This is why your internal time is most sensitive to and impacted by blue light.
- Your SCN sets your circadian rhythm when it shares your internal time with the rest of your body.
- Your SCN is indirectly connected to many areas of your brain that produce your stress hormone cortisol (in the adrenal gland), your sleep hormone melatonin (pineal gland), growth hormone (pituitary gland), and reproductive hormones (gonads) among others.
Your SCN controls your circadian rhythm. And with this, it also controls and optimizes the timing of many functions that are vital for your daily life.
We have seen above that you weaken your circadian rhythm if you are not getting enough light during the day and too much in the evening and night. Now have a look at the quick fact from above again. With special attention at the last line about the hormones connected to your SCN.
Here is what a weakened circadian rhythm means for your body: During the evening you produce too much cortisol (to keep you energized for the “day”) and not enough melatonin (to help you sleep and improves your sleep quality). And in the morning you don’t produce enough cortisol (to get you up and ready) and still too much melatonin (that keeps you sleepy).
As a result, you have a hard time falling asleep in the evening and won’t get a night of fully restorative sleep. And the next morning you have a hard time waking up and are feeling less energized during the day.
Discover more in this post about “Why Is the Circadian Rhythm so Important for You.”
Let’s take this to the extreme now. What would happen if your SCN doesn’t work anymore as it should?
Let’s take the case of patients that have a damaged SCN. Alzheimer patients in their later stages. Once their SCN has degenerated, they lose their circadian rhythm. And with this, they lose their sense of time and have great problems keeping a regular sleep-wake cycle. And anything that is connected to this. Their body functions that once had a specific time within their circadian rhythm now happen at random times. Ouch.10,11
Time for a little summary!
So far we know that light is the main component to set your circadian rhythm. Especially those blue light rays. We know that this is received by your melanopsin receptors in your eye. Those then translate your light information and send it to your SCN in your brain. And that your SCN uses this information to set your internal time. Finally, your circadian rhythm is the optimization of your daily functions and works based on your internal time.
But how does your SCN use light to adjust your circadian rhythm?
Synchronization of Your Circadian Rhythm
How Your Body Adjusts Your Circadian Rhythm
To start, your SCN needs external light information to adjust your circadian rhythm. This is because your circadian rhythm is not exactly twenty-four-hours long (have a look at the full story here). But your external day is.
To synchronize your circadian rhythm with the external day, your SCN (your internal master clock) uses the light information it receives from your melanopsin (those light receptors at the back of your eye) about the sun time of day. It constantly makes small adjustments to your internal clock. This process is called “light entrainment”.
How Light Entrainment Works
So, light entrainment is the name for when your SCN adapts your circadian rhythm to align it with your external day. And it does so through light signals it receives from your melanopsin about your light environment.2,12
Let’s go a little deeper and see what that exactly means for you.
Whatever light information your SCN receives, it has three basic options to adapt your circadian rhythm to the external day:
- Your SCN can speed up your internal clock – this is called the compression of your circadian rhythm
- Your SCN can leave your internal clock as it is
- Your SCN can slow down your internal clock – this is called the expansion of your circadian rhythm
Your circadian rhythm is like a pendulum, constantly swinging between night and day. You can picture this like a swing: Your SCN is the person behind pushing. And your circadian rhythm is tightly sitting on the swing.
Now, when the swing is at its starting point (in the morning), pushing it moves it forward. Your SCN speeds up your circadian rhythm. But after the swing has reached its endpoint and is on its way back (in the evening), pushing it goes against its movement. And your SCN slows down your circadian rhythm. Or even stops it.
Just like in this swing example (left picture), the impact of light on your circadian rhythm depends on its timing (right picture).
Now, let’s take a look at the right picture. Do you remember the three options that your SCN has to adapt your circadian rhythm? To speed it up (compression), to leave it as it is, and to slow it down (expansion). Let’s take those options into account and see how light influences your internal time – and with this also your circadian rhythm, which follows your internal time:
- Light in the first half of your twenty-four-hour day speeds up (compresses) your internal time. It is your SCN recognizing that it is already daytime. The earlier in the morning it is (the higher the adaptation curve), the more it has to compress your internal time. Light early in the morning speeds up your internal time much more than light closer to midday.
- Light during midday has little or no effect on your internal time. This is no new information for your SCN: it expects light and there is light. Accordingly, it does not need to adapt your internal time.
- Light during the second half of your twenty-four-hour day slows down (expands) your internal time. It is like your SCN thinking that it is still daytime. The later in the evening it is (the lower the adaptation curve), the more it has to expand your internal time. Again, light in the evening slows down your internal time much more than light closer to midday.
Your SCN uses light information to synchronize your internal time with the sun time of day (light entrainment). Through speeding up (compression) and slowing down (expansion) of your internal time, it ensures that your circadian rhythm matches your external day.
Different Day Lengths
How Your Body Entrains for Different Day Lengths
If you have a circadian rhythm that is longer than twenty-four-hours, then your SCN compresses it more than it expands it. And if you have a circadian rhythm that is shorter than twenty-four hours, then your SCN expands it more than it compresses it.
Either way, your SCN adapts your internal time through light entrainment so that it on average matches your external day.
“Humans whose clocks produce exactly twenty-four-hour days are extremely rare – most human clocks produce longer days, which therefore have to be compressed.”Till Roenneberg
Lets’ have a look at a real-world example: you! If you are rather an evening person, have a look at the left column. And if you are rather a morning person, have a look at the right column.
|Here is how light entrainment works for you if you have a circadian rhythm that is longer than twenty-four-hours:||Here is how light entrainment works for you if you have a circadian rhythm that is shorter than twenty-four-hours:|
|If your internal clock is longer than twenty-four-hours, then your internal time will be later than the external time of day.||If your internal clock is shorter than twenty-four-hours, then your internal time will be faster than the external time of day.|
|Your internal midday comes after the external midday.||Your internal midday comes before the external midday.|
|This means that you have relatively more time exposed before your internal midday (your compression area) than the time after your internal midday (your expansion area).||This means that you have relatively less time exposed before your internal midday (your compression area) than the time after your internal midday (your expansion area).|
|As a result, you have a relatively longer first part of the day until midday that will be compressed (sped up). And a relatively shorter second part of the day after midday that will be expanded (slowed down).||As a result, you have a relatively shorter first part of the day until midday that will be compressed (sped up). And a relatively longer second part of the day after midday that will be expanded (slowed down).|
|Your internal day will be more compressed than expanded. This allows your circadian rhythm to match the external day length.||Your internal day will be more expanded than compressed. This allows your circadian rhythm to match the external day length.|
Now let’s connect this information with the impact of light I mentioned above. Do you remember that light intensity in your room is probably around 100 lux? And that natural light outside has intensities between about 1,000 lux and 150,000 lux?
Well, the stronger the light intensity is (also called your zeitgeber), the more effective it helps your SCN to either compress or expand your internal time. This means the more effective it is at aligning your circadian rhythm with your external day.
A strong enough zeitgeber (light intensity) is important for this alignment: Let’s say your internal day is longer than twenty-four-hours. With a strong enough zeitgeber, it will be compressed enough to match the external twenty-four-hour day. But if your daily zeitgeber is too weak, then it will allow your days to be too long. The weaker the zeitgeber, the relatively longer – and more out of synch with the external day – your circadian rhythm will become.
And if your internal day is shorter than twenty-four-hours? Then a daily zeitgeber (light intensity) that is too weak will allow your internal days to be too short. The weaker the zeitgeber, the relatively shorter – and more out of synch with the external day – your circadian rhythm will become.
Let me be a bit nostalgic here and introduce you to a time when we still lived by sun time. And how that changed with the introduction of a universal time system.
From Sun Time to Social Time
How We Went From Being Aligned With Sun Time to Living by Social Time
In nature, you can see animals wake up and go to sleep based on the sun. Plants open their leaves aligned with when the sun comes out, then follow it during the day, and finally close it again when the sun goes down. Even single cells behave in synchrony with the sun.
Up until the nineteenth century, that was also how we lived. In alignment with the movement of the sun. But then trains challenged everything. To be more precise, the introduction of their timetables that led to the introduction of international standard times.
In 1884, the representatives of twenty-six nations met in Washington DC and implemented a universal time system.13 They divided the world into twenty-four time zones and set time zero to the local sun time at the Royal Observatory in Greenwich, a place close to London.
This is the system we use today. For example, Pacific Time is also known as “GMT-8.” GMT stands for Greenwich Mean Time, and the number behind indicates the time difference. “-8” means that this time is 8 hours behind the time in Greenwich. Or that the sun needs 8 more hours to travel from Greenwich to this place. At least in theory.
Practically, countries could adopt any time they want, as long as everyone agrees to use it. This is what happened in China. China works based on Beijing sun time and the entire mainland territory is covered by one single time zone. The sun, on the other hand, needs slightly more than two-and-a-half hours to travel from Beijing to western China. If you were to wake up in western China at 8 am, sun time would only be about 4:30 am. And this is also what your internal body clock would tell you. So much about a good start into the day. Every. Single. Day.
Ok, as real as the example from one single time-zone in China is, it is still an extreme case.
But differences between sun time and local time also do exist in virtually all places in the world. This is most likely also true for you. Albeit on a smaller scale. Germany is a well-studied case for this, so let’s look at this example.
Real-World Case Study
A Real-World Case Study From Germany
So far I have told you about the impact that light has on your circadian rhythm. That it depends on its timing and intensity. And also that it depends on whether your internal day is longer or shorter than twenty-four-hours.
Remember that the length of your internal day does also influence whether your circadian rhythm will be later or earlier? This is called your chronotype.
Why am I introducing you to your chronotype now? Because this allows you to get a more direct insight into how light impacts your circadian rhythm. With a real-world case study from Germany.
Read all about your chronotype in this post:”What Are Chronotypes and How to Find Out Yours“
How Your Longitude Impacts Your Circadian Rhythm
Earth is a globe with a circumference of 360 degrees. And conveniently, it is also divided into 360 longitudes that specify the east-west position of each location (latitudes specify the north-south location).
You can see the sun make a complete turn around the globe every twenty-four-hours. That is every 1,440 minutes. If we divide those 1,440 minutes by the 360 east-west longitudes, then we see that the sun needs four minutes to pass from one longitude to the nest one.
Till Roenneberg and colleagues have the biggest chronotype database in the world (the Munich ChronoType Questionnaire) with close to 300,000 entries. Including detailed entries for Germany that they could slice based on their respective longitude. And that is what they did.14,15
Germany spans over nine longitudes (longitudes six to fifteen to be precise) at its widest point. That means that the sun needs thirty-six minutes to travel from the furthest point in the east to the furthest point in the west.
And take a guess, what was the average difference in chronotypes for these two locations? Yes! Thirty-six minutes!
Till Roenneberg summarized it best when he said:
“Chronotypes move with the sun, becoming later by, on average, four minutes per longitude from east to west.”Till Roenneberg
Oh yeah, and he also mentioned that we are no different than animals with respect to our body clock. We all entrain to sun time. At least when we are exposed to enough natural sunlight.
How the Size of Your City Impacts Your Circadian Rhythm
So far about the general trend. But what about all the light pollution we know cities have? Would there be a difference in your circadian rhythm between living in a rural area or a city?
Well, Roenneberg also had this information. And he compared the influence of your city size on your circadian rhythm. More specifically, on the timing of your circadian rhythm, called your chronotype. Here is what they found:14,15
- People living in rural regions have on average the earliest chronotypes and are most closely aligned to sun time (read: circadian rhythms that are more closely aligned with the external day).
- People living in cities of up to half a million inhabitants tend to be later chronotypes and less closely aligned to sun time.
- People living in cities of more than half a million inhabitants are on average the latest chronotypes and the least aligned to sun time.
Do you remember that most people, and most likely you, have a circadian rhythm that is longer than twenty-four-hours? And that you need a strong light exposure – your zeitgeber – to align your internal time to the external time of day? So, the weaker the zeitgeber is the less aligned is your internal time to external time. And that expresses itself in a longer circadian rhythm. Which is called a later chronotype.
What does that mean for you? The bigger your city is the more you need to make an effort to get enough natural light exposure to live aligned with your circadian rhythm. Try to get more sunlight during the day. Especially the first part of the day if your internal day is longer than twenty-four hours (otherwise the second half of the day). And try to limit your artificial light exposure during the evening and night. Especially from blue-light-emitting devices such as LED light bulbs or any screens.
Read all about it here in “How to Get Your Circadian Rhythm Back on Track.”
My Personal Experiences
I live in a city and mostly work indoors. This puts me at risk of not getting enough natural light during the day and too much artificial light during the evening and night.
I know that my circadian rhythm is longer than twenty-four-hours, which makes me a late chronotype. This means that I would have trouble falling asleep early enough and waking up early enough if my circadian rhythm is not aligned with the external day.
How do I align my circadian rhythm with the external day? The answer is light. The right kind of light at the right time of day.
In the morning, any light helps me feel ready to start the day. Especially natural sunlight or really bright indoor light. Those special kinds of light that are made as sun therapy and that promise a light shower of 10,000 lux.
With this light hitting my eyes, my melanopsin receptors (those that help sense the time of day through light) are fully triggered. They then tell my SCN (my internal master clock that centrally controls my circadian rhythm) that it is daytime. And my SCN gets my circadian rhythm and my body ready. This means, among other things, that it stops the production of my “sleep hormone” melatonin and ramps up the production of my “stress hormone” cortisol (the one that energizes me and is beneficial in the morning).
During the day I try to get as much sunlight as possible. Especially during the first part of the day as this speeds up (compresses) my circadian rhythm. And brings it back on track.
This could range from taking my laptop and work to an outside place, taking a walking break around the block, or going for a run. Either way, I love being exposed to high light intensities at that time. And I am noticeably feeling better after each time outside. Maybe that’s just the placebo effect because I know how beneficial it is. But I’d gladly take that too.
In the evening, I try to avoid anything blue-light-emitting. I do not want my circadian rhythm to be slowed down (expanded) as this would postpone the time I can fall asleep. And it would reduce my sleep quality through a delayed onset of melatonin production (the “sleep hormone”) and still produce relatively too much cortisol (which was beneficial in the morning but is detrimental in the evening).
The lamps in my flat are set to warmer tones. The screen of my mobile phone is set to night mode. And my laptop screen has a reduced blue-light-color spectrum through an app called f.lux. I also change my glasses to ones that have a slight blue-light-filtering coating.
The result? I do not have any issues falling asleep. And I am consistently getting a high quality, restorative sleep. I am waking up refreshed and full of energy. Without an alarm clock. And I also feel full of energy during the day. All thanks to some small habits that align my circadian rhythm with my external day.
Finally, there are six key takeaways that I want to share with you:
- Your circadian rhythm is controlled by your internal master clock – a tiny part in your brain called the SCN.
- Your SCN needs to adjust your circadian rhythm to match the outside world. And it uses predominantly light information from the outside world to adjust your circadian rhythm.
- Your SCN adjusts your circadian rhythm depending on when it gets the light signals:
- Light in the first half of your twenty-four-hour day speeds up (compresses) your circadian rhythm.
- Light during midday has little or no effect on your circadian rhythm.
- Light during the second half of your twenty-four-hour day slows down (expands) your circadian rhythm.
- During the day, you need strong light intensities to get a robust circadian rhythm. Especially natural light outdoors. Only getting indoor light intensities weakens your circadian rhythm.
- In the evening and night, your blue-light-emitting screens and LED light bulbs are strong enough to disrupt your circadian rhythm. They tell your brain that it is not nighttime yet. This also weakens your circadian rhythm.
- A weakened circadian rhythm does not allow your body to optimize your daily functions. One common sign of a weak circadian rhythm is if you are feeling too energized to sleep at night and not enough to wake up in the morning.
And now back to you: Do you already have a strong circadian rhythm? If not, how would you use this information to strengthen your circadian rhythm?
PS: If you found this information useful, spread the word and help those who would benefit most from it 🙂
- 1.NobelPrize.org -. The Nobel Prize in Physiology or Medicine 2017. The Nobel Prize in Physiology or Medicine 2017. https://www.nobelprize.org/prizes/medicine/2017/summary/. Published 2017.
- 2.Roenneberg T. Internal Time. Harvard University Press; 2012.
- 3.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
- 4.Provencio I, Jiang G, De Grip WJ, Hayes WP, Rollag MD. Melanopsin: An opsin in melanophores, brain, and eye. Proceedings of the National Academy of Sciences. January 1998:340-345. doi:10.1073/pnas.95.1.340
- 5.Panda S. The Circadian Code. Rodale Books; 2018.
- 6.Panda S. Melanopsin (Opn4) Requirement for Normal Light-Induced Circadian Phase Shifting. Science. December 2002:2213-2216. doi:10.1126/science.1076848
- 7.Freedman MS. Regulation of Mammalian Circadian Behavior by Non-rod, Non-cone, Ocular Photoreceptors. Science. April 1999:502-504. doi:10.1126/science.284.5413.502
- 8.Varadarajan S, Tajiri M, Jain R, et al. Connectome of the Suprachiasmatic Nucleus: New Evidence of the Core-Shell Relationship. eNeuro. September 2018:ENEURO.0205-18.2018. doi:10.1523/eneuro.0205-18.2018
- 9.Moore RY. The Suprachiasmatic Nucleus and the Circadian Timing System. In: Progress in Molecular Biology and Translational Science. Elsevier; 2013:1-28. doi:10.1016/b978-0-12-396971-2.00001-4
- 10.Mirmiran M, Swaab DF, Kok JH, Hofman MA, Witting W, Van Gool WA. Chapter 11 Circadian rhythms and the suprachiasmatic nucleus in perinatal development, aging and Alzheimer’s disease. In: Progress in Brain Research. Elsevier; 1992:151-163. doi:10.1016/s0079-6123(08)64570-7
- 11.Van Erum J, Van Dam D, De Deyn PP. Sleep and Alzheimer’s disease: A pivotal role for the suprachiasmatic nucleus. Sleep Medicine Reviews. August 2018:17-27. doi:10.1016/j.smrv.2017.07.005
- 12.Roenneberg T, Daan S, Merrow M. The Art of Entrainment. J Biol Rhythms. June 2003:183-194. doi:10.1177/0748730403018003001
- 13.Gibson Bros. -. International Conference Held at Washington for the Purpose of Fixing a Prime Meridian and a Universal Day. Gibson Bros., Printers and Bookbinders; 1884:212. http://www.gutenberg.org/files/17759/17759-h/17759-h.htm.
- 14.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
- 15.Roenneberg T, Kumar CJ, Merrow M. The human circadian clock entrains to sun time. Current Biology. January 2007:R44-R45. doi:10.1016/j.cub.2006.12.011