Daylight Association Fatigue 📱


As someone working on new digital book format and starting a new business in ebooks space I recently stumbled upon a claim on the Internet that sent me searching for answers. Again! Truth is out there style. 🤣 Not unusual, but I mean this particular claim was both amusing and confusing at the same time, so I had to find out.

In one of the communities that I lurk on, a very vocal group concluded that backlit displays can cause harm to the human eye whereas electronic paper of a reader does not. The elephant in the room was whether or not a screenreader with an E-ink screen is better for reading books than a tablet with an iridescent screen. Why so? And the arguments presented ranged from because blue light could hurt our eyes to not looking at the source of light directly to it’s almost like reading on paper and so on.

Unfortunately, none of the responses leading to the conclusion were correct. I’m not sure about the conclusion either.

So what is really happening here? Is staring at your mobile or iPad all day really going to hurt our eyes in some way? How does the human eye work? What is vision?

Does it matter if a surface is lit up from behind or reflecting ambient light?

Enter physics

Let’s start by looking at physics of a normal human eye first:


[ Image credits: www.nkcf.org]


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 (intensity) by expanding or shrinking and letting in only a ‘safe amount’ of light for us to process.

This is how a normal eye works.


[ Image credits: www.nkcf.org]


Unless we are forced to stare straight into the Sun or a really bright filament directly, our iris and photoreceptors are perfectly capable of handling the visible part of the EM spectrum quite safely and accurately. Down to a single photon of light!

As you can see from the diagram above the process of collecting light rays through a lens and projecting them on a retinal surface on the back of the eye is quite simple and it follows the simple laws of optical refraction. This mechanism is also the reason why human’s are said to have a simple type of eye for peripheral vision as opposed to say a dragonfly. To our eyes the nature of source of light does not matter as long as the incoming flux is safe and is from the visible part of the EM spectrum (see figure below). The light could be coming directly from an iridiscent screen of a tablet or being reflected off a surface naturally like a physical book or a building.

This solves the first piece of our puzzle.

That looking at the source directly or not looking at the source of light directly doesn’t mean anything. 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 EM spectrum (see figure below) and isn’t bright enough to close our pupils and eyelids, that light doesn’t affect how our eyes function. If it did, we’d have had mobile screens on market that were reflecting the content off a mirror so that people weren’t looking at the source of light directly.

So the next question is: does ‘blue-light’ emanating from a tablet or mobile cause strain on our eyes?

Can it degrade our eyesight in any way?

The short answer is no. The blue-light of the visible spectrum coming off of any source be it a tablet, a phone, a bulb, an LED TV or the gas burner doesn’t affect our eyes in any way. This lightrange has absolutely nothing to do with how our eyes work as shown in the diagram above. In fact we might be better off to read on a tablet or phone screen depending on the time of the day and place because with an ereader we are essentially dependent on ambient lighting, which may or may not be sufficient for the eyes.

So why does mobile or tablet screen appear to bother our eyes then?

Enter Pyschophysics

Our retina functions much like the film in a camera. It is responsible for capturing all of the light rays, processing them into light impulses through millions of tiny nerve endings, then sending these light impulses through over a million nerve fibers to the optic nerve that connects directly to our brain. The brain is where these impulses are processed giving us the ability called vision.

It is literally our brains that are ‘seeing’ and not our eyes!



During the day when we are awake our brain receives and processes a humongous amount of data from our eyes, ears, tongue and skin. To be awake, our brain uses a lot more energy than it does during the night when we are asleep. A part of this daytime activity is that our eyes are feeding a continuous stream of light impulses to our brain and this signal helps our brain to determine and protect us from situations like stumbling over or walking into other people on the street etc. The brain is operating at a higher energy level.

The surge of photons due to daylight (EM spectrum) raises our attention level and our brain is alert throughout the day and continues to function at a higher energy level, which is also exhausting, until the Sun sets in the evening. It is just the way our body works, in a cyclical circadian rhythym!

A tablet’s display (or phone) is similar to blue sky.

The spectrum of light that hits our cornea from a tablet (or phone) is similar to ‘daylight spectrum’ of morning. The cooler blue-end of the light spectrum grabs our attention, like early morning daylight, and pushes us to hang on to the screen attentively for more. This continues to happen in the night as well after sunset because our brain thinks that it is day! Since we continue to function at a higher energy (attention) level and our brain is unable to determine that the day has officially ended, we tire easily and our circadian rhythym is affected adversely setting in a fatigue. We’ll call this daytime association fatigue, or DAF for future references.

Note that our brains will not stop working until it is sure that the Sun has set and it is safe to go back to the bed. It’s in our biology.

To tackle DAF, there is an option of turning red-shift on which signals dusk on your tablet or phone. Apple calls this Night Shift which uses warmer colors in the evening according to the time of the day. Our brain can then ascertain evenings and a continued usage of the irridiscent doesn’t interfere with our circadian rhythm, and we can go back to sleep easily.

As you can see it is easy to confuse the influence of daylight spectrum (or morning-light or blue-light) on our circadian rhythym with the weakening of eyesight due to things like muscle (pupillary) atrophy, lens degradation or even retinal burn due to staring at high intensity light source. It is easy to mistake DAF for eyestrain and choose a wrong device to read longform on.

Chances are that if you are a night owl, you might be better off with a tablet instead of an ereader so that there is no dependence on potentially low ambient light. But if you are into reading a book on a beach in broad daylight, a reader with en E-ink screen or even a physical book is better. If you’re tired looking at an irridiscient screen after a day’s work, it is better not to pick up anything to read because your body needs rest already.

Great, so now you can choose a device for yourself with better reasons.

Feel free to throw in your science-hat in the ring if this article or my understanding of the subject needs improvement!


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! Well, may be not——but I do recommend reading a Superbook on your iPad. It’s magical on the iPad Safari! ⛷️