Daylight Energy Fatigue 🌤️


I recently stumbled upon a claim about Kindles (and other E-ink screen tablets in general) that sent me spinning and searching for answers. Here’s how it began:

The Claim(s)

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

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

Point being made here was whether or not a screenreader with an E-ink screen is better for reading books than a tablet with an iridescent screen. And why so.

And the arguments 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. No clear explanation was provided that was grounded in fundamentals of physics and tribalism soon drowned every other voice on the forum.

The Opportunity

I had to find out more.

As a developer working on new book format for the web I am always excited about new ways of printing or reading books online. It’s important to understand how things work. I try to keep up with the pace and go with a hope to understand what’s happening under the hood so that I can find a solution or suggest one to a friend with great confidence. This was a perfect opportunity for me to put my search hat on venture into the world of science for solid answers.


Unfortunately, none of the responses in favor of the claim were correct and even the claim itself isn’t well-grounded for the most part. There is absolutely no advantage of an E-ink type of screen (that’s on a Kindle )over an IPS panel iridescent display (that’s on an iPad) as long as the reader is inside a building and the room is well-lit to reflect enough light off of the surface of a screenreader so as to not strain the eyes of the reader.

Okay. But what is really happening here? Why does staring at your mobile or iPad all day wear us out? How does the human eye work? What is vision? Does it matter if a surface is backlit or reflecting ambient lighting?

Enter Physics

Let’s start with a look at a normal human eye first:

[ Image credits:]

The human eye is a simple optical receptor. Light rays enter our eyes through the transparent cornea which are then focused on to 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. Exposure.

This is how a normal eye works.

[ Image credits:]

Unless we were 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 light spectrum safely.

Down to a single photon of light.

From the diagram of a simple eye above, it is clear that the process of collecting light rays through a lens and projecting those rays on 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. The simplicity of this mechanism is also the reason why we are said to possess a simple type of eye for peripheral vision as opposed to say a dragonfly that has thousands of eyes per eyeball—or a compound eye.

What matters to our vision is thus simply the intensity of visible light and nothing else.

The source of light could be anything, it could be a light bulb or an iridiscent screen of a phone or a wall or a painting or a surface of a physical book or even a Kindle that reflects light off of a night lamp or another source. Reflected or direct, it doesn’t matter as long as the incoming flux or intensity of light is safe for our pupillary filters to allow.

This solves the first piece of our puzzle:

That looking at the source directly or indirectly means nothing. If you’re looking at the wall, the wall IS the source of light for your eyes. Even though the light rays may have originated millions of miles away on the surface of the Sun.

This leaves us with another possibility that if a room is not well-lit, the intensity of reflected light off of a Kindle may not be enough or healthy for your eyes in the long run.

Conclusion 1:

If we are looking at something, we are looking at the source of that light directly. Always. Now the light itself could be reflected or radiated (originated) by the body in question but as long as it is in the visible part of the light, the electro-magnetic spectrum, and it isn’t too bright for our pupils and eyelids to close involuntarily, that light doesn’t affect how our eyes function. If it did, we’d have had reflector screens on market that were reflecting the content off an iPad so that the we didn’t have to look at the source of light directly.

¯\_(ツ)_/¯ Yes.

Claim 2.

So the next claim (number ii) to evaluate is ‘because blue light could hurt our eyes…’

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

To understand this let’s take the full electro-magnetic spectrum of light into account. Lights of different types can be specified with the following eight letters: G X U V I M R P, where the letters stand for:

| 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.) that have a very high frequency are on the extreme left end of the spectrum whereas the power frequencies (P.) that have a very high wavelength are on the extreme right end of the spectrum.

Notice how small 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 shown above, also known as the VIBGYOR part of the em-spectrum:

The blue light is represented by the letter ‘B’ on the VIBGYOR and lies between 380 to 500 nm. Then there is indigo and then there is violet light before we move into the left ultra-violet section 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–but it doesn’t affect our eyes. The visible lightrange has absolutely nothing to do with how our eyes work. In fact we may be better off reading on a tablet or a phone depending on the time of the day and how well-lit a place is because with a screenreader we are largely dependent on ambient lighting, which may or may not be sufficient for the eyes.

But wait, does not a mobile or tablet screen bother our eyes much? Yes, it does. Let’s look at why 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.

Not being able to decide when the day has officially ended affects our circadian rhythym and can lead to screen fatigue by the time we fall back to 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.