Dark or white color theme is better for the eyes?

What is better for the eyes, a dark color theme or a white color theme?

Answer

There has been a lot of research on this topic since the 1980s and a lot of it still holds true today. One study from the 1980s states this:

However, most studies have shown that dark characters on a light background are superior to light characters on a dark background (when the refresh rate is fairly high). For example, Bauer and Cavonius (1980) found that participants were 26% more accurate in reading text when they read it with dark characters on a light background.

Reference: Bauer, D., & Cavonius, C., R. (1980). Improving the legibility of visual display units through contrast reversal. In E. Grandjean, E. Vigliani (Eds.), Ergonomic Aspects of Visual Display Terminals (pp. 137-142). London: Taylor & Francis

The reason why this matters is because of focus. As this article on UXMovement states, "white stimulates all three types of color sensitive visual receptors in the human eye in nearly equal amounts." It causes the eye to focus by tightening the iris. Since the eye is focused, dark letter forms on light backgrounds are easier to read. When using a dark background with strong light letter forms, the iris opens to allow more light in, but that causes letter forms to blur. Why?

People with astigmatism (approximately 50% of the population) find it harder to read white text on black than black text on white. Part of this has to do with light levels: with a bright display (white background) the iris closes a bit more, decreasing the effect of the "deformed" lens; with a dark display (black background) the iris opens to receive more light and the deformation of the lens creates a much fuzzier focus at the eye.

Jason Harrison – Post Doctoral Fellow, Imager Lab Manager – Sensory Perception and Interaction Research Group, University of British Columbia

Now there seem to be varying factors into contrast and legibility. Room ambient lighting. Brightness of the monitor. Also you can mitigate the straining effects of white (#FFF) on black (#000) by simply lessening the contrast like using a light gray (#EEE, #DDD, #CCC) on a dark background (#111, #222).

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Update (Feb 7, 2020):

A new article from the Nielsen Norman Group entitled, "Dark Mode vs. Light Mode: Which Is Better?", brings some more research to this topic. A couple key findings in the article:

Cosima Piepenbrock and her colleagues at the Institut für Experimentelle Psychologie in Düsseldorf, Germany studied two groups of adults with normal (or corrected-to-normal) vision: young adults (18 to 33 years old) and older adults (60 to 85 years old). None of the participants suffered from any eye diseases (e.g., cataract).

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Their results showed that light mode won across all dimensions: irrespective of age, the positive contrast polarity was better for both visual-acuity tasks and for proofreading tasks. However, the difference between light mode and dark mode in the visual-acuity task was smaller for older adults than for younger adults — meaning that, although light mode was better for older adults, too, they did not benefit from it as much as younger adults, at least in the visual-acuity task.

The research did find though that dark mode seemed to be beneficial for users with impaired vision:

In Legge’s study, each of the 7 participants with cloudy ocular media had better reading rates with dark modes, whereas the rest of the participants, who had impaired central vision, were not affected by contrast polarity.

Though they did note one caveat that this study used CRT displays instead of LEDs displays.

A few takeaways from the article:

1. In general, light mode leads to better performance most of the time for users with normal or corrected-to-normal vision.


2. While light mode performs better, those gain seem to be more short-term. Long-term exposure may be result in myopia.


3. Increased font-size in dark mode doesn't offset the gains from light-mode.


4. Providing a dark mode though is still recommended though becomes of the potential long-term effects with light mode, some visual impairments perform better in dark mode, and some users simply prefer it.


5. For applications which provide long-form reading (books, articles, even news sites), dark mode options are recommended.

One other note with the studies cited in the article is that the studies focused on "glanceable" reading (i.e. reading 1-2 words on a mobile phone, smartwatch, or car dashboard).

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Further reading: