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Dark Mode is the marquee feature of macOS 10.14 Mojave. Apple even gives it the top spot on the macOS product page, saying:

Dark Mode is a dramatic new look that helps you focus on your work. The subtle colors and fine points of your content take center screen as toolbars and menus recede into the background. Switch it on in the General pane in System Preferences to create a beautiful, distraction-free working environment that’s easy on the eyes—in every way. Dark Mode works with built-in apps that come with your Mac, and third-party apps can adopt it, too.

And third-party apps have adopted it in droves. For months after Mojave shipped, our Watchlist items dutifully reported that this app and that app now supported Dark Mode or had tweaked their support for it in some important way.

Have you ever wondered why, if Dark Mode is such a revelation, it took Apple 35 years after the first Macintosh to revert to the look of the light-on-dark CRT-based monitors of the Apple ][ and IBM PC era? Were those green-on-black and amber-on-black screens really so wonderful?

No, they weren’t, and one of the Mac’s most significant design decisions back in 1984 was an interface that put black text and graphics on a white screen, just as had been done in print for hundreds of years. This wasn’t exactly an innovation since the designers of the experimental Xerox Alto (which Apple’s engineers copied liberally) also chose to display text as black on a white background. Going even farther back, Douglas Engelbart’s “Mother of All Demos” of the NLS also used a black-on-white display. But the Macintosh was the first mainstream personal computer whose screen tried to mimic paper.

Unfortunately, Apple’s marketing claims about Dark Mode’s benefits fly in the face of the science of human visual perception. Except in extraordinary situations, Dark Mode is not easy on the eyes, in any way. The human eyes and brain prefer dark-on-light, and reversing that forces them to work harder to read text, parse controls, and comprehend what you’re seeing.

It may be hip and trendy, but put bluntly, Dark Mode likely makes those who turn it on slower and less productive. Here’s why, if you adopted Dark Mode purely because Apple promoted it as the new hotness, you should think hard about switching back to the Light Mode that your eyes and brain prefer in System Preferences > General.

Dark-on-Light: The Background

Vision research has shown that humans prefer dark-on-light. That’s because, in the real world, the background of any scene around you is usually bright. Humans evolved outside, and we are generally active during the daytime and asleep when it’s dark. What we care about are the objects in front of the background, whether food, tools, predators, or whatever. Those objects are by definition darker than the background because they’re illuminated by the sun, or indoors, by whatever lights may be on. Even light-colored objects stand out from a bright background because they’re illuminated from some direction other than precisely behind you. That makes for indirect illumination, putting much of the object in shadow and thus darker than the background.

This is, of course, an oversimplification. Backgrounds are not always brighter than objects in front of them (think of a campfire at dusk), and indoor lighting can be much more variable. But the general rule holds—the background is generally brighter than the objects in a scene, and so the human brain becomes much more used to dark objects against light backgrounds and thus prefers them. This preference may even be hard-wired into our brains. Three-month-old babies shown images with both light-on-dark and dark-on-light components look toward the latter first.

For an example, let me reach into our archives. In “Better than the Printed Page: Reading on an iPad” (15 March 2018), Charles Maurer illustrated this preference with a couple of overly colorized pictures of Abraham Lincoln. Both use bizarre colors to eliminate any sense of familiarity with one or the other, but you’ll find that the right-hand image is intrinsically easier to identify. (And yes, since faces are easier to recognize than most other objects, this image is meant to illustrate, not prove the point.)

(I’m referencing Charles’s articles for TidBITS here because his wife Daphne Maurer is an experimental psychologist at McMaster University and a prominent visual scientist who was recently named a Fellow of the American Association for the Advancement of Science (AAAS) for her ground-breaking research on the development of vision in human infants. Charles has worked extensively with Daphne over the years, and they are in the midst of preparing a book about perception, which is why I couldn’t persuade Charles to write this article himself.)

Applying Dark-on-Light to Screens

Images like Technicolor Abe above aren’t really in question here—we’re mainly talking about text, which is made up of thin lines. When text is white on a black background as it would be in Dark Mode, the whiteness of the lines lightens the edges of each line broadly on both sides, blurring the edge. If the thin lines of the text are black and the background is white, however, white from both sides washes over the entire line, lightening it evenly, so the edges aren’t blurred. Charles illustrated this for me with a photo of a cactus backlit by the sun, in which you can see the bright optical flare making the cactus lighter in front of it.

Blur is a bad thing because of how the human eye relies primarily on contrast when extracting detail from an image. In “Reality and Digital Pictures” (12 December 2005), Charles wrote:

The eye does not see light per se, it sees changes in light – contrast. If two objects do not contrast with one another, to the eye they meld into one. This fact makes controlling the contrast of adjacent details to be paramount in importance.

He was focused on issues revolving around photographs, but contrast has been shown to be paramount in numerous studies of textual legibility as well. Of course, contrast goes in both directions—black on white and white on black both have high contrast. In the scientific literature, black on white is called “positive polarity,” whereas white on black is called “negative polarity.” Numerous studies over decades of research have found that positive polarity displays provide improved performance in a variety of areas. (While early studies used CRTs, all recent studies rely on LCD-based displays.) To quote from the introduction of a 2013 paper by Piepenbrock, Mayr, Mund, and Buchner in the journal Ergonomics:

For instance, a positive polarity advantage has been found in error rates and reading speed in a letter identification task (Bauer and Cavonius 1980), the number of transcribed letters onto paper (Radl 1980), subjective ratings on visual comfort (Saito, Taptagaporn, and Salvendy 1993; Taptagaporn and Saito 1990, 1993), text comprehension (A. H. Wang, Fang, and Chen 2003), reading speed (Chan and Lee 2005) and proofreading performance (Buchner and Baumgartner 2007). Taptagaporn and Saito (1990, 1993) tracked changes in pupil size for different illumination levels as well as for the viewing of different visual targets, such as a cathode ray tube (CRT) display, script and keyboard. They found less visual fatigue as measured by the frequency of changes in pupil size when working was accomplished with a positive than with a negative polarity display. Likewise, Saito, Taptagaporn, and Salvendy (1993) found faster lens accommodation and thus faster focusing of the eye with positive than with negative polarity displays.

To summarize, a dark-on-light (positive polarity) display like a Mac in Light Mode provides better performance in focusing of the eye, identifying letters, transcribing letters, text comprehension, reading speed, and proofreading performance, and at least some older studies suggest that using a positive polarity display results in less visual fatigue and increased visual comfort. The benefits apply to both the young and the old, as that paper concludes:

In an ageing society, age-related vision changes need to be considered when designing digital displays. Visual acuity testing and a proofreading task revealed a positive polarity advantage for younger and older adults. Dark characters on light background lead to better legibility and are strongly recommended independent of observer’s age.

In another study published in the Journal of the Human Factors and Ergonomics Society that focused on how positive display polarity helps when reading small text in small font sizes, Piepenbrock, Mayr, and Buchner concluded:

The implications seem important for the design of text on such displays as those of computers, automotive control and entertainment systems, and smartphones that are increasingly used for the consumption of text-based media and communication. The sizes of these displays are limited, and it is tempting to use small font sizes to convey as much information as possible. Especially with small font sizes, negative polarity displays should be avoided.

Since Apple has now announced that iOS 13 will introduce a Dark Mode similar to Mojave’s, you’ll want to avoid it there too or take a performance and productivity hit.

Screen Brightness and Ambient Brightness

There’s an obvious caveat to the comment about the human eye preferring dark objects against a light background. Apart from a few exceptions like fire, lightning, and bioluminescent fireflies, almost nothing in the natural world emits light.

In our modern world, however, screens do emit light, and quite a lot of it. (There’s truth in advertising here—many, if not most modern-day screens are lit from behind by LEDs, or light-emitting diodes. Those that aren’t use fluorescent lamps instead.)

So what, then, is the role of both screen and ambient brightness in the question? From a comfort standpoint, there’s no question that a bright white screen can be glaring and hard to view. Paper reflects about 90 percent of whatever light strikes it and is thus always slightly dimmer than the surrounding light, but an iPhone or iPad screen, for instance, is often brighter than the surrounding light sources.

When there’s a mismatch between the two—the screen is too dim outside or too bright inside—it’s hard to look at. That’s why Apple implemented automatic brightness control in iOS (find it in Settings > General > Accessibility > Display Accommodations) to reduce the screen brightness when you’re reading in a dark bedroom and increase it when you’re trying to take a picture on a sunny day. Apple’s algorithms might not fit what your eyes prefer, but it’s easy to adjust the brightness manually in Control Center if necessary.

Similarly, Apple’s True Tone technology on many devices tries to adapt the display to the colors in the surrounding environment to reduce the harshness of glancing from a warmly lit room to a cold white iPad screen. Keep True Tone enabled in Settings > Display & Brightness (iOS) and System Preferences > Displays (macOS, when supported by hardware) for most situations.

Notice that I haven’t said anything about all the cognitive benefits of a dark-on-light positive polarity display with respect to light in the surrounding environment. That’s because the research suggests that ambient light is irrelevant. In another paper in Ergonomics, Buchner and Baumgartner showed that the benefits of positive polarity displays were independent of ambient light when they compared results of the same experiment run in a darkened room versus one with typical office lighting. (Nor did chromaticity—blue and yellow as opposed to black and white—make a difference. It’s all about positive polarity, baby.)

So while night-owl programmers can say that they find Dark Mode more comfortable during stints of nocturnal coding in unlit rooms, they might finish sooner if they stick with traditional dark text on a light background (while keeping the brightness of the light background appropriate for the room lighting, of course).

Go Toward the Light

All that said, there are always people who are outliers. For instance, sleep research suggests that nearly all adults need 7–9 hours of sleep per night, but about 1% of people naturally sleep less. (Most of those who say they need less sleep are actually operating as though they were 8 years older than they actually are.) In much the same vein, I’m sure there are some people for whom Dark Mode is legitimately better, perhaps due to vision problems like floaters or light-triggered migraines. But for the vast majority of people, the science is pretty clear—Dark Mode can hurt your productivity.

There are also excellent niche uses for Dark Mode. Let’s say you’re a musician who uses a MacBook Pro as part of your performance on a darkened stage. Even though the MacBook Pro will likely be harder and slower to use in Dark Mode, those downsides are probably worthwhile to avoid a glowing white light illuminating your face. Similarly, if you read at night on your iPhone while someone else is sleeping next to you, it’s only kind to switch to light-on-dark—whether in an ebook app or in iOS 13 if it does gain Dark Mode—to reduce the chance your insomnia will wake up your bed partner.

Finally, everyone is, of course, welcome to make their own choices concerning Dark Mode. We all make decisions about what we prefer even when research suggests those decisions may not be optimal.

But whatever you choose, it’s important to understand that Apple’s marketers don’t have any science backing up their promotion of Dark Mode. The fact that macOS has long had an Invert Colors option in System Preferences > Accessibility > Display for those whose vision requires it suggests that Dark Mode exists largely because it has somehow become trendy.

Use it if you wish, but know that it’s a productivity hit for most people most of the time.