Solving Sudoku at Night: Dark Mode and Screen Brightness
Hiroki Obara · Developer
In 2007, Buchner and Baumgartner at Heinrich Heine University Düsseldorf published a study in the journal Ergonomics on whether the contrast direction of a screen changes how well people proofread. Across a series of experiments, proofreading was consistently better with dark text on a light background than with light text on a dark background. The difference held whether the room was dark or lit like a typical office.
Dark Mode on a phone sits on the light-text-on-dark side. This post goes through the research on screen polarity and the brightness settings on iPhone, with the case of solving Sudoku on a screen at night in mind.
Dark on light, light on dark
Researchers call dark text on a light background “positive polarity” and light text on a dark background “negative polarity”.
Besides proofreading scores, Buchner and Baumgartner measured breathing rate, heart rate and skin conductance, and asked participants to rate their mood, fatigue, eyestrain, headache and muscle strain. None of these varied with the display condition.
In 2013, Piepenbrock and colleagues at the same university compared younger and older adults in a study. Participants took a visual acuity test and did a proofreading task, either with black on white or white on black. Both age groups did better with dark characters on a light background.
A 2014 study by Piepenbrock and colleagues looked at why, by measuring pupil size. The idea they tested is that a positive polarity screen is brighter overall, so the pupil gets smaller, the image on the retina gets sharper and fine detail is easier to see. Pupils were indeed smaller and proofreading was better with dark text on a light background. The authors wrote that the results fit this explanation and discussed the study’s limits.
All of these results come from tasks where people read text on a screen and looked for errors.
Light Mode and Dark Mode on a tablet
In 2025, Sengsoon and Intaruk at Walailak University in Thailand published a study comparing Light Mode and Dark Mode on a tablet. The participants were 30 women aged 18 to 25. In each mode they used a tablet for one hour, 30 minutes playing a game and 30 minutes reading a novel, and filled in questionnaires on visual fatigue and dry eye before and after. Screen brightness was set to 100 percent, and room lighting was kept between 450 and 500 lux.
Visual fatigue scores went up after an hour in both modes, with no significant difference between the modes. Dry eye scores also went up in both modes, and the rise was slightly smaller in Dark Mode: on average 7.6 points in Light Mode and 6.4 points in Dark Mode.
The participants were young women, and the results describe one hour of use in a well-lit room.
Brightness and Dark Mode on iPhone
Apple’s iPhone User Guide page “Adjust the screen brightness and color on iPhone” covers brightness, Dark Mode, Night Shift and True Tone.
You can change brightness by dragging the brightness control in Control Center up or down, or with the slider in Settings, under Display & Brightness. With Auto-Brightness turned on, in Accessibility under Display & Text Size, iPhone uses its ambient light sensor to adjust the screen to the current light. The guide also notes that dimming the screen extends battery life.
The guide describes Dark Mode as “a dark color scheme that’s perfect for low-light environments”, and says it lets you use your iPhone “while, for example, reading in bed, without disturbing the person next to you.” You can choose Dark under Display & Brightness, or touch and hold the brightness control in Control Center to switch it. If you turn on Automatic and pick Sunset to Sunrise under Options, Dark Mode comes on only at night.
Night Shift shifts the colors of the display to the warmer end of the spectrum at night. Under Display & Brightness, then Night Shift, you can turn on Scheduled and use the Color Temperature slider to make it warmer or cooler. True Tone adapts the color and intensity of the display to the light around you.
Contrast direction and brightness
The 2014 study tried to explain the advantage of dark text on light through the brighter screen and its effect on the pupil. On iPhone, Dark Mode, the brightness slider, Auto-Brightness and Night Shift are separate settings, each set on its own.
Some people switch to a dark screen at night and a light one during the day. Others keep the same look all day.
For break guidelines when looking at a screen for a long time, see Are Tablets Bad for Children’s Eyes?. Research on screens before bed and sleep is in Brain Training in the Morning or at Night.
About Sudoku365
Sudoku365 is a Sudoku app for iPhone, iPad and Android. It uses the classic 9×9 board. It has four difficulties, Easy, Medium, Hard and Expert, with 300 puzzles each, 1,200 in total. There is a Daily Challenge with one puzzle a day and a streak, and two people can play head to head or together over Bluetooth.
Under Theme in Settings, you can choose System, Light or Dark. It starts on System, which follows your device’s light or dark setting. If your phone switches to Dark Mode at night, the board turns dark at night too.
The app is free with ads, and a one-time Remove Ads purchase turns them off. For your first puzzle, see Getting Started with Sudoku365.
