Smartphones, Viewing Distance (2017, 2008) & Asthenopia
Learn more about eye fatigue.
There have been a number of studies about how mobile device use impacts sleep due to the fact the the blue light radiation from smart phones inhibits melatonin production, which in turn inhibits sleep.
Researchers did an experimental study investigating the relationship between sleep, the distance the user holds the phone from their eyes, and eye fatigue.
In a small sample of nursing students the researchers used a subjective measure of quality of sleep and physically measured the distance between the smartphone and the users' head. The distance was measured for subjects both lying down and sitting up.
The amount of the viewing distance is directly related to the amount of eye fatigue, or asthenopia. Consequently it was expected that the viewing distance would also be related with the quality of sleep.
They noted that the viewing distance ranged from 5 1/4" to almost 13" for students who were using smartphones while sitting up. Users who were lying down tended to have a shorter viewing distance (3 3/4" to about 8 1/2").
Those subjects with the shorter viewing distance had poorer sleep and sleep efficiency.
The recommendation therefore is that a longer viewing distance causes less eye strain than a shorter viewing distance, and especially as it impacts sleep.
Researchers: M. Yoshimura, M. Kitazawa, et al
Published: Smartphone viewing distance and sleep: an experimental study utilizing motion capture technology, Nature and Science of Sleep, March, 2017.
This study evaluated the degree of eyestrain in people reading a novel excerpt from a smartphone for 60 minutes. All of the people read the same material.
The subjects were young, healthy and had normal vision. The 60 minute period was divided into six 10 minute sections and viewing distance was measured by taking a photo of the subjects every minute.
Interestingly, the viewing distance was greater for the 1st and 2nd 10 minute periods, shorter for the 3rd and 4th, and longer for the 5th. It was as though the person reading realized after about 40 minutes that they were holding the phone too close to their eyes and their eyes were getting tired.
The viewing distances began at about 10-14" and gradually decreased to 9-13" in the 2nd 10 minutes. It decreased additionally in the 3rd and 4th 10 minute periods, but increased slightly to 8-13" in the 5th period.
As would be expected the symptoms of tired eyes, and vision becoming blurry were markedly greater after the 60 minute period.
The only symptom that correlated with a change in viewing distance was identified by the users as 'uncomfortable eyes.' In other words, regardless of viewing distance, there was more eye fatigue after an hour, but for those holding the smartphone further away, the eyes felt more comfortable.
Researchers: J. Long, R. Cheung, et al
Published: Viewing distance and eyestrain symptoms with prolonged viewing of smartphones, Clinical & Experimental Optometry, March, 2017.
Researchers developing the earlier mobile devices investigated on how readable characters were - these were on mobile phone liquid cystal displays.
The research is relevant in the context of eye fatigue in that they found that younger users held the phones closer to their eyes than older subjects. In short, younger users with excessive smartphone use will have more vision problems and eye fatigue than older users simply due to the fact that they tend to hold the phones closer to their eyes.
Researchers: S. Hasegawa, K. Fujikake, et al
Published: International Journal of Occupation Safety and Ergonomics, 2008.