Daylight Energy Fatigue. 🌤️

Daylight Energy Fatigue. 🌤️

  • superbooks
  • 9 minutes
  • March 7, 2019

I recently stumbled on the following claim on the interwebs:

“…that backlit displays can cause harm to the human eye whereas an electronic paper type of screen on a screen reader does not.”

The user here was trying to up-sell the idea that a screen reader with an E-ink screen is better suited for long-haul reading than a tablet with an iridescent IPS panel screen.

The justifications they recommended included reflectivity of a tablet screen, glare, or a form of ‘blue light radiation’ emanating from a backlit display. As is usual on discussion forums online, the thread led to plenty of heated arguments, with each influencer advocating for why they thought [the product name here] was better for their eyes:

  1. Not looking at the source of light directly,
  2. Because blue light could hurt our eyes,
  3. Because e-ink is almost like reading on paper and therefore…

For a moment, I thought that if the paper was so much better than a digital screen and if it had all these magical qualities for our eyes, then why aren’t people moving back to the dead-tree?

You know, line up textbooks on a shelf made of teakwood. 🤯

The quick answer

To cut a long story short, none of the arguments for or against an e-ink screen over an iridescent one are accurate or reliable. If you look at both the products through the lens of science, each has an advantage over the other depending on when and where you take-up reading.

If your lifestyle is to sit on a beach and read under the Sun, you are probably better off with an e-ink screen. However, if you are a night-owl who reads under a blanket, then a self-illuminating tablet is better.

As long as the flux of light (intensity) is good for your eyes to discern the text without straining itself, your vision will not be affected by the underlying technology in any way.

However, if the ambient light is low, then there is a chance for your eyes to be hurting more with a screenreader than a tablet.

As in, a tablet is safer to read on indoors than an e-reader.

Now you might be thinking…

Wait how?

Why does using a smartphone or an iPad all day feel so tiring then?

Does it really not matter if the screen is backlit or the reflecting type?

Enter the human eye!

To answer the questions above, let us take a close look at the human eye:

[ Image credits:]

A human eye is a simple optical receptor.

Light rays enter from one end through a transparent cornea and are then focused on to the other end, on the back of the eye, onto the retinal photoreceptors using a crystalline convex lens in the middle.

The iris and the pupil take care of the intensity, i.e., the incoming flux of light, by expanding or shrinking the aperture to let-in only a safe amount of light for our photoreceptors to process.

This process of control of ‘incoming light rays’ by the eye is called exposure, and it works exactly the way exposure works on a camera.

This is how a normal healthy eye works.

[ Image credits:]

Excess light from a bright source like the Sun can burn our retinas, whereas a dull source can increase the effort required to read or process the context ahead of us, thereby lowering vision over long periods.

Unless we are paralysed in the eye and end up staring straight into the Sun with no safety in place, our iris and pupil are perfectly capable of handling the intensity of light safely.

Down to a single photon of light. Yes, it is that sensitive.

In other words, the process of collecting light through a lensing eyeball and projecting it onto a retinal surface on the back of the eye is a simple process of optical physics. It follows the general laws of classical physics using a mechanism is called optical refraction.

What matters to us, for the safety of our eyes, is only the intensity of the light and nothing else. Too dim or too bright are both harmful for us.

What does not matter to our eyes is what the source of light is or its distance. The incoming light could be coming from a primary source like a bulb or a phone or a secondary source like a wall reflecting the light of the bulb or the ceiling or simply off a page on the book.

The nature or physics of light does not change no matter what the source is (Sun, mirror, or paper) or where the light originated (whether it is ten feet away or a million miles away like the Sun or the Moon) before reaching our eyes.

As long as the intensity (or flux) of light is safe for our pupillary filters to pass through, we can stare at that source of light as long as we want.

This solves the first piece of our puzzle:

Puzzle one:

That looking at the source of light directly (or indirectly) does not matter to our eyes. Only the intensity of light does.

Based on the principle of optical physics or the nature of light, it is clear that an e-reader has absolutely no advantage over a tablet since looking at the source of light directly or looking at an object that reflects the light in the room is of no consequence to the eye.

The only thing that matters is whether the object in front of us is too bright or too dull for the eyes to focus. Is the intensity of light hitting our eyes safe?

One way to test this conclusion is by looking at the reflection of an iPad on your bathroom mirror. You are no longer looking at the source of light directly now. Does it change the way content appears to you on the iPad?

If reflection had mattered, there would have been reflectors available on the market reflecting content on the iPad so that we did not have to look at the shining screen directly. ¯\_(ツ)_/¯.

What is that with the ‘blue-light’ thing then?

Another line that gets thrown around on the web often is that a ‘blue-light could hurt our eyes…’. I realize that this statement is so common online that it is virtually impossible to right the buyer’s view at this point.

“Love my Kindle Oasis. Had a Paperwhite before that was also great, but the Oasis lets you use a reddish light instead of blue.”

The claim that the blue-colored light may harm the human eye is often passed on to suggest that a product-X is better than a product-Y.

This claim is so outrageous that a little in-depth research is required to dispel the myth. To clear through all the marketing-speak and understand the mechanisms at play from a scientific standpoint.

Let us start with a look at the “full spectrum of light” first. Light is an electromagnetic spectrum that can be categorized into the following broad categories:

| Alphabet    | Type or light            |
| ----------- | ------------------------ |
| G.          | Gamma rays               |
| X.          | X-rays.                  |
| U.          | Ultraviolet zone         |
| V.          | Visible spectrum         |
| I.          | Infrared zone.           |
| M.          | Microwaves              |
| R.          | Radio waves              |
| P.          | Power frequencies        |

The gamma rays (represented by letter G) lie on the extreme left end of the spectrum, and Power Frequencies (P.) sit on the extreme right end of the spectrum.

Notice how only a tiny sliver of the entire spectrum is visible to our eyes!

The visible part, V., is merely a narrow range of frequencies that our photoreceptors can perceive. This range of frequencies is also known as the VIBGYOR part of the em-spectrum:

Assuming that the ‘blue-light’ mentioned by the users online is also represented by the letter ‘B’ of the VIBGYOR, let us look at how this blue-light affects us in everyday life.

Blue gets the most attention…

Blue is in the middle of the VIBGYOR spectrum alongside green (G) and yellow-colored lights (Y). These colors (blue, green, and yellow) are perhaps the most commonly found ones in nature. We soak in a world of blue, green, and yellow throughout our lives!

Naturally, we love our blue skies and yellow Sun, and a green planet.

Blue-light is at the front-center part of the visible spectrum, and VIBGYOR is exactly how our day progresses from sunrise to sunset.

In the morning, at daybreak, when the sky turns from deep black of the night to a shade of blue, the world around us lights up, and the morning shade of blue ushers a new day.

Our brain releases cortisol, and we wake up and start receiving a humongous amount of data or light impulses through the optic nerve. (And from other sensors or receptors in the body.)

Naturally, we start using more metabolic energy to process the incoming data through our sensory system, and our brain starts operating at a higher energy level for the day. It is like a computer’s CPU where a heavy process can kickstart the fan. Daylight raises our senses and pushes our brain to be more attentive to our surroundings.

This goes on throughout the day until the sunsets, and the evening (warm end of the spectrum) prompts us to go back to sleep again.

It is just the way our body works.

In a cyclical circadian rhythm from night through the day.

Our brain and its receptors are in a state of rest at night when we are sleeping (sleep mode), but during the day, the brain is pushed to a higher state of consciousness.

How does the blueness of morning matter to us then?

A modern iridescent screen of mobile or tablet works exactly like a morning sky. Its spectrum is similar to the light of a morning.

When we are exposed to blue light, cortisol rises because our brain thinks that this blue light is the same as morning light from the sun. The range of wavelengths hitting our cornea from a tablet or mobile screen is similar to the one that we experience at the break of dawn.

It helps us gather our senses and be attentive to the context at work or office, just as one would be with a sunrise.

Process the information quickly.

However, we continue to look into the same screen way into the night, and our brains are thus tricked into thinking that it is still daytime.

We are hooked to the morning-light spectrum coming off of our mobile phones instead of the Sun, and this confuses the brain into not detecting the day is officially over. Our brain forces us to remain alert and operate at a higher energy level even though we are tired in the evening.

Not being able to discern when the day has officially ended, our brain loses its circadian rhythym[1]. This leads us to fatigue by the time we actually retire to bed.

Let us call this fatigue ‘Daylight Energy Fatigue’ or DEF due to lack of referenceable literature on the subject. Please do correct me if the issue has been identified before and labeled differently elsewhere.

Avoiding the DEF

To avoid DEF, there is now an option of turning red-shift on your smartphone. Apple calls this feature the Night Shift, which shifts the colors on the screen spectrum towards the warm end of the evening (red-shift and not red-color). This change in the device’s color temperature prompts our brain to think that the daily close is near.

It is time to put the phone down and go back to sleep.

People can easily confuse DEF with eyestrain, which is a completely different issue. Eyestrain deals with actual medical conditions like pupillary muscle atrophy, or milky lens, or focal degradation, or a retinal burn by staring down a high-intensity light source.

These conditions have nothing to do with how mobile phones or tablet screens work and might need proper medical attention.

DEF merely fatigue of the mind.


Well, now that you know that the blue-light or the red-light does not affect your eyes, but only your sleep cycle, you can safely choose a tablet or an e-reader according to your lifestyle.

If you are a night owl, you might be better off with a tablet, but if you are a daytime reader who prefers reading outside in the Sun, then a reflective e-reader or a physical book is a better option.

Other than that, there is no difference between the two screens.

If you are tired after your day at work sitting in front of a computer, do not pop open a physical book to read because your mind and the body need good rest.

[1] Updated on Dec’ 18, 2020: Thanks to a hat-tip by a friend on Twitter, I learned that this topic was under active research until recently. Research on discoveries of molecular mechanisms controlling the circadian rhythm led professors Michael Young, Michael Rosbash and Jeffrey C. Hall to win the Nobel Prize for medicine in 2017. Just missed! 😜

About the author

Marvin Danig

I write code with my bare hands. 💪🏻 Yammer about Bubblin all day.