DEF or Daylight Energy Fatigue 🌤️

I recently stumbled on a claim by the experts of Kindle (and of other E-ink screen tablets possibly as well) that sent me spinning and searching for answers a bit. Here’s how it began:

The Claim(s)

In one very vocal online communities out there, a splinter group claimed the following:

That …backlit displays can cause harm to the human eye whereas an electronic paper screen of a screenreader cannot.

Point being made here by this user was that a screenreader with an E-ink screen is better for reading books than a tablet with an iridescent screen or in other words an IPS panel.

The argument was followed up with reasoning and the community raged from (i) not looking at the source of light directly to (ii) because blue light could hurt our eyes to (iii) it’s almost like reading on paper and so on. However, no clear scientific explanation was provided that was grounded in fundamentals of physics.

Tribalism soon drowned every other voice on the forum and decided to logout and search for answers myself.

The Opportunity

As someone working on new book format for the web I had to find out.

Besides, I am always excited about new ways of printing, publishing or reading books online. This was a perfect opportunity for me to put on my search ninja hat on venture into the world of science and technology for some solid answers.

The Findings

As one would suspect, none of the arguments or counter-arguments in favor or against the benefits of an E-ink screen are accurate and even the claim itself of a screenreader being healthier for the eyes than an iridescent screen isn’t grounded on science for the most part.

There is generally no advantage of an E-ink type of screen that’s on a Kindle (say) over an IPS panel iridescent display that’s on an iPad (say) depending on ambience of your surroundings. If your lifestyle and the ambient lighting is okay, meaning as long as the room that you are in is well-lit to reflect enough light off of the surface of a screenreader so as to expose the text clearly and not strain your eyes for discerning them in anyway, you are in a good situation.

But if the ambient lighting is low, then you are probably hurting your eyes more than helping it with a screenreader. With an IPS panel on an iPad however that dependence on ambient lighting is done away with.

So what is really happening here? Why does staring into your mobile or iPad all day feel strenuous after sometime? Does it really not matter if a surface is backlit or reflecting ambient lighting?

Short answer is: No, it doesn’t matter if a surface is backlit or frontlit. If your eyes are feeling tired already, give yourself some rest. Let’s look at the long answer now.

Enter Physics

Here’s a diagram of normal human eye:

[ Image credits:]

The human eye is a simple optical receptor.

Light rays enter our eyes from one end, through a transparent cornea which are then focused on to the other end, the retinal photoreceptors on the back of our eye using a crystalline convex len in the middle. The iris and pupil take care of the incoming flux of light—the intensity—by expanding or shrinking and letting in only a safe amount of light for our photoreceptors to process. That’s called exposure, like the one in your camera.

This is how a normal eye works.

[ Image credits:]

Unless we are forced to stare straight into the Sun for a considerable amount of time, our iris and photoreceptors are perfectly capable of handling the visible part of the spectrum safely.

Down to a single photon of light!

From the diagram above, it is clear that the process of collecting light rays through the lens and projecting those light rays onto a retinal surface on the back of the eye is quite simple and it follows simple laws of physics i.e. optical refraction through a convex lens. This simplicity of the mechanism is also a reason why humans are said to possess a simple type of eye for peripheral vision as opposed to (say) a dragonfly that has thousands of tiny eyes per eyeball—or a compound eye.

What matters to our eyes is thus only the intensity of light and nothing else.

The source of light could be anything. It could be a primary source of light like an incandescent bulb or an iridiscent screen of a phone or a secondary source of light like the surface of a wall or a painting or a page on a physical book— it doesn’t matter. Reflected or direct, light is the same and its nature doesn’t change. As long as the intensity of incoming light is safe for our pupillary filters to allow we can stare at the source as long as we wanted.

If you are looking at the wall, the wall IS the source of light for you even though the rays may have originated elsewhere millions of miles away on the surface of the Sun. With a secondary source like a wall however, there is a chance that if it is not well-lit, the intensity of reflected light off of the surface isn’t going to be enough for the eyes to read in.

This solves the first piece of our puzzle:

Conclusion 1:

That looking at the source directly or indirectly means nothing.

If it did, we’d have had reflector screens on market that were reflecting the content off an iPad so that our eyes didn’t have to look at the source of light directly. ¯\_(ツ)_/¯ simply.

Claim 2.

The next claim was ‘because blue light could hurt our eyes…’

This one is so deceivingly outrageous that a little research was required to nip the issue in the bud. The first question to ask here is: what does ‘blue light’ mean here? What does ‘blue’ stand for in the vast spectrum of light?

Below given is the full specification of an electro-magnetic spectrum of light. Light rays of different types are categorized according to the following eight letters: G X U V I M R P, where each letter stand for the following:

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

The gamma rays (G.) on the extreme left end of the spectrum have a very high frequency are while the power frequencies (P.) on the extreme right of the spectrum have a very high wavelength.

Notice how tiny a portion of the complete specturm can our eyes really see? The visible part (i.e. V.) is barely a narrow sliver in the middle of the spectrum as shown above. This is also known as the VIBGYOR part of the EM-Spectrum:

By assuming in good faith that the ‘blue light’ mentioned by the user on the forum means the blue light represented by the letter ‘B’ on VIBGYOR (the range between 380 to 500 nm) we can see that there is indigo on the left of blue, then there is violet light further left before we move into the ultra-violet region of the spectrum.

So, can this ‘blue light’ emanating from a mobile screen or a bulb or a torch affect our eyes in anyway?

The short answer is: No.

The bluelight in the visible part of spectrum coming off of any source–be it a tablet, a phone, a bulb, an LED TV or even the gas burner–doesn’t affect our eyes in any way. In fact the visible lightrange has absolutely nothing to do with how our simple eyes function—see the process of refraction above. It has something to do with our brains, but certainly not the eyes.

Let’s look at that next.

Enter Pyschophysics

Our retina functions much like the film in a camera. It is responsible for capturing all of the light rays coming through the eyeball, processing them into little light impulses through millions of tiny nerve endings, and then sending those light impulses over a million nerve fibers through the optic nerve directly into our brain.

The brain then processes these impulses giving us the acuity called vision.

I mean, it is literally our brains that are seeing and not our eyes!

Every morning, when the day breaks, we wake up and our eyes open up to daylight by slowly letting in a flood of light from the outside. The brain starts receiving this humongous amount of data (light impulses) coming through the optic nerve along with data from other receptors too, like the auditory nerve and the touch sensors under our skin that had been in a state of rest at night.

The brain uses a lot more energy during the day than it does at night when we are asleep. As part of this daytime activity, our eyes feed a continuous stream of light impulses to our brain and this signal can be cut-off or focused on by our brain to determine and protect us from situations like stumbling over or walking into other people on the street etc.

In essence we operate at a higher state of consciousness when we are awake.

The surge of photons of daylight (the natural EM spectrum of sunlight) raises our attention level and the brain remains alert at a higher energy state throughout the day until the Sun sets in the evening prompting our bodies to begin rest for the night.

It is just the way our body clock works, in a cyclical circadian rhythm.

Light from a modern iridescent screen (IPS panel) on a mobile or tablet is similar to the morning light spectrum of daytime. The range of light that hits our cornea is similar to the lightrays of dawn. It is the blue shift of the morning light that grabs our attention and raises the dopamine level of our brain.

This helps us to concentrate and focus on tasks during the day at work.

However, when the same process continues well into the night, even after the Sun has set, our brain continues to think that it is still daytime because we continue to recieve the daylight spectrum off of the mobile screen. As long as we scour for more information off the interwebs, our brain continues to function at a higher energy level which is exhausting. Note, it has nothing to do with the eyes though except in that the thick datastream of light impulses is tiring our brain further.

By not being able to discern when the day has officially ended, affects our circadian rhythym and can lead to screen fatigue by the time we are supposed to fall sleep. We are going to call this Daylight Energy Fatigue, or DEF, for lack of referenceable literature on the subject. Please correct me if I’m wrong in my research.

DEF Never

To tackle DEF there is now an option of turning red-shift on on your smartphone or tablet, which can help signal the brain that the day is now over. Apple calls this feature the Night Shift which uses the warmer colors of the daylight spectrum (representing evening) to prompt end of day. A continued usage of the iridiscent screen with red-shift on doesn’t affect our circadian rhythm and we can go back to sleep normally after a day’s worth of run.

It is easy to confuse this influence of daylight spectrum (morning-light or blue-shift) on our circadian rhythym with the weakening of eyesight due to possibilities like pupillary muscle atrophy, lens degradation or even retinal burn by staring at high intensity light source—which are different medical conditions that need separate treatments. DEF is often mistaken for eyestrain and we end up choosing a wrong device to read longform on.


Chances are that if you are a night owl, it might be better for your eyes with a tablet instead of a screenreader simply because there is no dependence on potentially low ambient lighting. But if you prefer reading outdoors, a screenreader with en E-ink screen or even a physical book may be better—the third argument claimed by the group earlier.

Overall, if you are tired after your day job, it is better not to pick up anything to read because your brain needs to rest to remain healthy.

Great. Now we can all go back to arguing what is better for each one of us with some more understanding.

Written by: Marvin Danig, CEO of Bubblin Superbooks. Want to follow me on Twitter?

P.S.: It’s likely that some of you read this post on your desktop. That’s obsolete! We recommend the iPad. It’s magical.