Why Is Blue Light Both Good & Bad for You and Your Sleep

Why Is Blue Light Both Good & Bad for You and Your Sleep

Why Is Blue Light Both Good & Bad for You and Your Sleep

Your body constantly optimizes all your functions based on the time of day. But how does your body know what to do when? It depends on your internal time for that. And to align your not-completely-precise internal time to your external always-twenty-four-hours-long day, it uses light cues from your environment. To be more precise, it especially uses the perceived intensity of blue light.

Why is blue light both good and bad for you and your sleep? The effect blue light has on you depends on its timing. Natural light that is high in blue light in the first part of the day has a positive effect on you and your sleep. Artificial blue light (especially from screens and LEDs) in the evening and at night has a negative effect on you and your sleep.

Read on to get a full understanding of:

  • What role blue light plays for you
  • Why you should get more blue light during the day
  • Why you should avoid blue light at night
  • How blue light impacts (digital) eye strain
  • My personal experiences and your key takeaways

Blue light has a special function for your body. Its function is to align your internal time to your external time. This is why blue light can be both good and bad for you, depending on the timing you are exposed to it.

The Role of Blue Light

What Is the Role of Blue Light for Your Body

To understand the role blue light has for you – and also to see when it is good and when it is bad for you – we need to have a look at why it is so special for you.

Here is a quick summary of why blue light plays such a special role for your body:​1–3​

  • Your body cannot perform all your functions at the same time, it optimizes all of them over the course of every day and night.
  • This time-based optimization is done based on the internal time your body tracks and called your circadian rhythm.
  • But your internal time is not exactly twenty-four hours long and needs to be aligned to the external day. This happens through your light exposure.
  • You have special receptors at the back of your eye (called melanopsin) that help your body sense the external time (and they don’t help you see).
  • Those receptors are directly linked to the part of your brain (called the SCN) that is in charge of your central internal time and circadian rhythm.
  • And those receptors are especially sensitive to blue light, while they are not that much triggered by warmer light tones.

In short, blue light helps your body align your internal time with your external time. And you have special light receptors for this that don’t help you see but that help your brain sense the external time. Blue light activates these 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.

To get a visual understanding of this, have a look at the following collage of the midnight sun. Can you see the difference in light color that sunlight has over the course of a day (and night here)?

Now, if you could choose which light-color your eyes should be most sensitive to figure out when the day starts, which would you choose? Yes! The blue-light colors! And that is exactly what happened during evolution.

You are adapted to natural sunlight. And your internal clock needs natural sunlight to align the length of your internal day to your external day.

You can read all about it here in: “How Does Your Circadian Rhythm Work

Now, sunlight has two main factors that change over the day. One is how much blue-light is present. And the other is how intensive the light is (the perceived light intensity is measured in Lux).​4,5​

Natural sunlightDuring the dayIn the evening/ at night
Blue light raysHighVirtually not present
Light intensity~ 10.000 – 150.000 Lux< 1 Lux

As you can see in the table above, there is a stark difference during the day and in the evening/ at night. And while this difference is vast if you are outside and exposed to natural light, it is a completely different story if you are inside and exposed to artificial light.

So, let’s compare this to a typical light environment inside of a building, like your house or office.​4,5​

Artificial lightDuring the dayIn the evening/ night
Blue light raysHighHigh
Light intensity~ 50 – 500 Lux~ 50 – 500 Lux

As you can see from this table, artificial light doesn’t differentiate between day and night. Well, that was to expect because it is artificial after all. But what is more interesting is the differences in the presence of blue light and light intensity.

Remember that we evolved in the presence of natural sunlight. And, not surprisingly, we also need both the presence of blue light and the light intensities of sunlight to set our internal time correctly.

But when you spend time indoors, this is what your internal clock registers:

  • During the day, your internal clock expects a lot of light and also a lot of blue light. Your screens and LEDs normally can’t match these intensities.
  • During the evening and night, your internal clock expects hardly any light and no blue light. Your screens and LEDs easily go beyond those intensities.

You need blue light to set your internal clock. And your internal clock is the basis for your body to optimize all your functions – including your sleep-wake behavior. 

Your body expects a lot of blue light during the day and next to nothing in the evening and at night. This is also the reason why the effect of blue light on you depends on its timing.

Your everyday (blue) light exposure most likely isn’t intensive enough to tell your brain that it is daytime. And then you still receive too much blue light for your brain to think that it is nighttime. This weakens your circadian rhythm. It prevents your body from getting ready for both day and night. 

Let’s have a look at this next to see what that practically means for you.

Blue Light During the Day

Why Should You Get More Blue Light During the Day

During the day is the time when blue light is good for you. And when you ensure that you get enough blue light, you also set yourself up for a strong circadian rhythm. This, in turn, allows you to tap into a high sleep quality.​4,5​

Let’s start with an overview of the light exposure your body expects (sunlight) and the light exposure your body most likely gets (artificial light):

During the daySunlight (outdoors)Artificial light (indoors)
Blue light raysHighHigh
Light intensity~ 10.000 – 150.000 Lux~100 – 500 Lux

The problem is that the light intensity you receive during the day is most likely not high enough. Unless you spend a few hours a day outside. But if you, like most people, spend most of your time indoors, then you only get a fraction of the light intensity that your body expects. And with this, you also only get a fraction of the blue light that your body expects.

The result? Your body can’t properly align your internal clock. It expects many thousand Lux full of blue light but only receives a few hundred. This weakens your circadian rhythm and doesn’t allow your body to optimize its functions over your day and night.

Let’s have a concrete look at what this means for you. Let’s have a look at the most prominent expression of your circadian rhythm: your sleep-wake cycle.

There are two key hormones that your circadian rhythm controls to establish your sleep-wake cycle. Melatonin, which helps you sleep and improves your sleep quality. And cortisol, which keeps you energized for the day (also known as your “stress hormone”). 

Now, if you regularly don’t receive enough blue light during the day, then your body can’t optimize the upregulation (cortisol) and the downregulation (melatonin) of these hormones. Your body (your circadian rhythm) can’t prepare you to wake up. You don’t produce enough cortisol yet. And still too much melatonin. The too-low levels of cortisol prevent you from being as energized as you should be. And the too-high levels of melatonin still keep you sleepy. This combination gives you a hard time waking up and getting ready for the day – both physically and mentally.​6,7​

To get a strong sleep-wake cycle you need a strong circadian rhythm. And this means that you need the right light intensities at the right time. You need to get plenty of blue light during the day. (Remember that you have those receptors in the back of your eyes that are especially sensitive to blue light and help your brain set your circadian rhythm?)

Have a look at this post about “Why Is the Circadian Rhythm so Important for You” to get a deeper understanding.

What can you do during the day to help your body optimize all your functions? Get plenty of natural light – that is also rich in blue light – during the day. In general, the biggest problem at this time is the light intensities, to try to increase them as much as possible.

Here are four practical tips on how you can increase your (blue) light exposure during the day:

  1. Spend at least two hours outdoors during the first half of every day​8​
  2. Stay as close to windows as possible if you are indoors​9​
  3. Take artificial light showers during the day if needed​10​
  4. Don’t wear blue-light filtering glasses or sunglasses​5​

Have a look at tips #1 – #4 in the post “How to Get Your Circadian Rhythm Back on Track” for the full information.

Blue Light at Night

Why Should You Avoid Blue Light in the Evening and at Night

In the evening and at night is the time when blue light is bad for you and your sleep. And when you are exposed to too much blue light (e.g., through any screens or LEDs), then your body thinks that it is still daytime and doesn’t fully prepare you for the night. This weakens your circadian rhythm and you will have a relatively hard time falling asleep and a lower sleep quality.​4,5​

Let’s start with an overview of the light exposure your body expects (sunlight) and the light exposure your body most likely gets (artificial light):

In the evening/ at nightSunlight (outdoors)Artificial light (indoors)
Blue light raysVirtually not presentHigh
Light intensity< 1 Lux~100 – 500 Lux

The problem is that the blue light intensity you receive during the evening and at night is most likely too high. Thanks to electricity you can stay with light long after the sun has set. While you wouldn’t want to change that, your body might think differently – at least about the light environment you put yourself in. The problem is that the screens of all of your devices, like your smartphone, your TV or your eReader, and also modern LEDs emit a lot of blue light.

Any blue light that is present means for your body that it still can’t be night. The melanopsin receptors at the back of your eye register any blue light above a certain threshold. And you easily cross that threshold the blue light emission of the screens of your devices and also your LEDs.

The result? You weaken your circadian rhythm and your body is not able to optimize your functions for the night.

Let’s have a concrete look at what this means for you. Let’s have a look again at the most prominent expression of your circadian rhythm: your sleep-wake cycle. And the two key hormones melanopsin (that helps you with sleep) and cortisol (that energizes you).

Now, too much blue light at night means that your body can’t optimize the downregulation (cortisol) and the upregulation (melatonin) of these hormones. It can’t prepare you to fall asleep. You still produce too much cortisol. And not enough melatonin yet. The too-high levels of cortisol still keep you more energized than you should be. And the too-low levels of melatonin don’t prepare you enough to fall asleep. This combination gives you a hard time falling asleep and reduces your sleep quality.​6,7​

To get a strong sleep-wake cycle you need a strong circadian rhythm. And this means that you need the right light intensities at the right time. You need to cut down the amount of blue light that you are exposed to in the evening and at night. (Remember that you have those receptors in the back of your eyes that are especially sensitive to blue light and help your brain set your circadian rhythm?)

Have a look at this post about “Why Is the Circadian Rhythm so Important for You” to get a deeper understanding.

What can you do in the evening and at night to help your body optimize all your functions? Cut back on your time in front of screens. And change the blue-light emission from your LEDs. In general, the biggest problem at this time is that you expose yourself to too high blue light intensities.

Here are four practical tips on how you can increase your (blue) light exposure during the day:

  1. Reduce blue light emissions from your screens​11,12​
  2. Adapt your home lighting​5​
  3. Use blue-light-filtering glasses​13​
  4. Avoid any light (late) at night if you can​14​

Have a look at tips #6 – #9 in the post “How to Get Your Circadian Rhythm Back on Track” for the full information.

Blue Light and Eye Strain

How Blue Light Impacts (Digital) Eye Strain

You have seen how blue light is beneficial for you during the day. And how it is detrimental for you during the evening and at night. But what about the artificial blue light that your devices, like your computer or mobile phone, emit during the day? And is there any connection between these and (digital) eye strain?

If you were going to your optician, I could bet that they’d try to upsell you on blue-light-filtering glasses. Their claim? Those glasses are vital if you work with screens and they prevent digital eye strain. Because blue lights are the reason for your eye strain and their glasses block that blue light. So much about their marketing. But what does science say?

There is limited evidence that digital eye strain is caused by the blue lights emitted from the screens of your devices.​15​ There are many potential causes for digital eye strain and blue lights were just hypothesized as one of them.​16​

While the associated symptoms of digital eye strain, such as eyestrain (obviously), but also headaches, blurred vision or dry eyes are more than frustrating, blue-light filtering glasses are not the quick fix against it. At least not beyond the placebo effect.

As you have seen above, a great time to use blue-light-filtering glasses is in the evening and at night. At that time, all blue lights are detrimental to you because they then interfere with your circadian rhythm and weaken it. So, if you by chance have blue-light-filtering glasses simply don’t wear them during the day but in the evening and at night instead.

Personal Experiences

My Personal Experiences

Personally, I can feel a massive difference between (blue) light exposure during the day and in the evening/ at night. Maybe that is simply because I am aware of how light affects my body at different times. Either way, I’m happy to get that relatively direct feedback from my body and try to act on it too.

In the mornings, I try to go out pretty much as soon as I wake up. I love it running in the early morning and getting as much sunlight as possible. And on the days that I don’t run, I simply take a short ten to fifteen-minute walk.

I can literally feel my tiredness melt away as I’m outside. And my energy levels go up too. And the more sunlight there is, the better I feel.

But I can really feel the big impact this morning light has on me during the days when I don’t go outside in the morning for an extended time. On those days, I just don’t feel as energized as normal. And it is relatively harder for me to concentrate and stay focused on tasks.

In the evenings and at night, I limit my blue light exposure as much as possible. I have blue-light-filtering glasses that I wear at these times and all my screens automatically reduce their blue lights based on the time of day. In addition, I also reduce their brightness.

The crazy thing is the immediate difference that I can feel. This becomes especially evident to me when I have to look at screens that are not reduced in blue lights. It then feels as if my brain is fired up again. While this might be great to concentrate a little longer at night, I can feel a massive reduction in my sleep quality. And it takes that much longer for me to fall asleep.

That’s why I always try to maximize my natural light exposure during the day and minimize my blue light exposure in the evening and at night.

Key Takeaways

Key Takeaways

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

  1. Blue light helps you to align your internal time with your external time. The expression of your internal time is your circadian rhythm that controls and optimizes all the functions in your body.
  2. During the day is the time when blue light is good for you. And when you ensure that you get enough blue light, you also set yourself up for a strong circadian rhythm.
  3. In the evening and at night is the time when blue light is bad for you and your sleep. And when you are exposed to too much blue light, then your body thinks that it is still daytime and doesn’t fully prepare you for the night. This weakens your circadian rhythm.

And now back to you: How would you rate your daily blue light exposure? Are you getting enough natural blue light during the day? And do you ensure that you are not getting too much artificial blue light in the evening and at night?

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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    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
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    Avery DH, Kizer D, Bolte MA, Hellekson C. Bright light therapy of subsyndromal seasonal affective disorder in the workplace: morning vs. afternoon exposure. Acta Psychiatr Scand. April 2001:267-274. doi:10.1034/j.1600-0447.2001.00078.x
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    Green A, Cohen-Zion M, Haim A, Dagan Y. Evening light exposure to computer screens disrupts human sleep, biological rhythms, and attention abilities. Chronobiology International. May 2017:855-865. doi:10.1080/07420528.2017.1324878
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    Kimberly B, James R. P. AMBER LENSES TO BLOCK BLUE LIGHT AND IMPROVE SLEEP: A RANDOMIZED TRIAL. Chronobiology International. December 2009:1602-1612. doi:10.3109/07420520903523719
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Hi, I'm Dennis

The content of every post is based on peer-reviewed, published studies combined with my own experience of translating those theories into real-life practice.

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