In the earliest days of home computing, the much-hyped feature we now call “dark mode” was the default. Cathode ray tube (CRT) monitors, also known as monochrome monitors, got their _Matrix_-esque green-on-black look from the phosphor-coated inside of the screens.

It’s been a long time since manufacturers stopped painting monitors with phosphor, but some people still swear by dark themes for everything. The Verge keeps a running list of apps and platforms that offer a dark mode for fans, and Apple announced a dark mode feature for iOS 13 at the company's annual WWDC conference this week.

"It’s thoughtfully designed to make every element on the screen easier on your eyes," Apple claimed in its announcement of the feature. But there’s a wide array of human experience and physiology that keeps that from being true for everybody. For some people with astigmatisms (a common condition that most people are born with, to some degree), dark mode can be worse on their eyes, and, impairments that require it aside, the science showing that dark mode is any easier on the eyes than normal mode is still up for debate.

For a lot of users with vision impairments or disorders, dark mode offers a better experience and allows them to use technology they otherwise may not have been able to. Flipping the standard black-text-on-white color scheme has long been an accessibility option for Apple computers. But there’s little evidence suggesting that using dark mode during the day, in a brightly-lit office or on your phone, is better than the alternative for the vast majority of people.

Apple's announcement page for the dark mode feature.

That dark mode is healthier for your eyes has become a bit of folk wisdom. Wired called dark themes "an eye-friendly alternative to the traditional blindingly bright user interfaces," and Popular Science called them a "comforting alternative to the blinding white" of most websites. Experts say that's not proven, however.

"I do not think dark mode affects eye health in any way given the data that is out there in the literature," Euna Koo, an ophthalmologist at the Stanford Byers Eye Institute, told CNN Business. "The duration of use is likely much more important than the mode or the intensity of the brightness of the device when it comes to the effect of this dark mode on eye fatigue and potentially eye health."

When we’re talking about the potentially damaging effects of white-background screens, we’re usually really talking about “blue light,” part of the light spectrum made of short, high-energy wavelengths. A 2018 study published in BMJ Open Ophthalmology notes that blue light could be a factor in eye tiredness, but cites dry eyes from not blinking for long periods as a more serious cause of eye strain, as well as too-small fonts, and conditions like uncorrected astigmatism.

A 2018 study by researchers at the University of Toledo claimed that blue light could contribute to macular degeneration, and plenty of news outlets ran with that claim, writing that our phones are blinding us. But the American Academy of Ophthalmology denounced that study, because the researchers didn't study real eye mechanisms, or cells taken from eyes.

Studies have shown that blue light from screens at night can mess with circadian rhythms and make it harder to get quality sleep, but whether blue light from our phones is causing eye strain is still up for debate.

There is a physiological reason why black text on a white screen is better for many people. I have severe astigmatism, meaning my corneas are irregularly-shaped, causing my vision to be a little blurry all of the time, no matter how strong my contact lenses are. Most people have astigmatism to some degree, but it's usually unnoticeable, according to the American Optometric Association.

But when an astigmatism is bad enough to impair vision, light text on dark backgrounds aggravates the condition, making text harder to read—and therefore making people squint more to try to correct it. As Gizmodo wrote in 2014, citing research by the Sensory Perception and Interaction Research Group, at University of British Columbia, white backgrounds act as a "crutch" for astigmatic eyes:

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.

My own very-astigmatic eyes are exhausted by dark mode, but for many others, dark themes are an accessibility benefit. White backgrounds emphasize floaters, those tiny spots of fibers that appear in some people's vision. People with disorders like photophobia or keratoconus, conditions that cause high sensitivity to light, might read more easily with dark themes.

Dark themes are also commonly seen as less distracting, especially for platforms like Spotify and Steam: The designers want the outside world to fall away, so you spend more time in the app. That may also be why programmers use dark backgrounds for long coding sessions, to focus more easily on each line.

On one point, I must concede: Dark mode is good for battery life on the latest iPhones. According to iFixit, on OLED screens—which light pixels individually, meaning turned-off pixels don't use any power—the swaths of black areas in dark themes conserve battery. LED screens light all pixels from the edges of the screen and use the same amount of power whether the screen is black or white. The newest iPhones have OLED screens, but any models before the iPhone X uses a LCD screen.

In the end, more display options are better, and people should use whichever lighting theme they want. It’s great that dark mode is coming to iOS for people who it helps, but there’s simply not evidence to make the blanket claim that dark mode is “easier on your eyes.”