Dark Mode and Astigmatism: Why White Text Glows, and What Fixes It

Short answer: with astigmatism, dark mode often makes text harder to read, not easier. The fix, in order: get the astigmatism corrected, use light mode in a lit room, and if you keep dark mode, lower its contrast and add room light.

What halation looks like

• Light text on a dark background looks fuzzy, as if each letter has a faint glow around it.
• Thin strokes smear, or seem to have a ghost copy beside them.
• The smear has a direction — horizontal, vertical or diagonal — rather than spreading evenly.
• The same text in black on white looks sharper.

If that matches, the cause is almost certainly optical, in your eye, not in the display.

Why dark mode does it

In astigmatism, the cornea or the lens of the eye is shaped slightly more like a rugby ball than a football. It focuses light more strongly along one direction than the other, so a point of light lands on the retina as a short streak. That is why the smear has a direction.

Two things make this worse in dark mode:

1. Bright on dark shows the blur. In light mode the blur is still there, but it spreads from a bright background into dark letters, where you hardly notice it. In dark mode the bright letters spread into a dark background, and the spread is visible as a glow.
2. A dark screen opens the pupil. Less light reaches the eye, so the pupil widens. A wider pupil uses more of the cornea and lens, including the less regular edges, so the blur gets bigger. A narrow pupil works like a pinhole and sharpens the image. That is also why squinting helps.

This is common. About 40% of adults have measurable astigmatism, and a large minority have enough — a dioptre or more — to see halation clearly. Many of them have never had it corrected, because it does not bother them in daylight.

Two checks you can do now

The squint test

Open a page with white text on a black background. Squint until your eyes are nearly closed, or look through a tiny hole made with your curled finger. If the text gets noticeably sharper, the blur is optical. The squint narrows the opening in the same way a smaller pupil does.

The head-tilt test

Look at the same text and slowly tilt your head to one side. If the direction of the smear turns with your head instead of staying fixed to the screen, the smear is coming from your eye. Astigmatism has an axis, and the axis moves with you.

Neither check is a diagnosis. Both tell you that an eye exam is likely to help more than a new theme.

What the evidence says about dark mode

Research on display polarity finds that, for most readers in a normally lit room, dark text on a light background is read faster and more accurately than the reverse. The advantage of light mode is larger for small text. Eye-care bodies, including the American Academy of Ophthalmology in its patient material, describe dark mode as a comfort preference rather than a treatment for eye strain.

Buchner, A. & Baumgartner, N. (2007). Text–background polarity affects performance irrespective of ambient illumination and colour contrast. Ergonomics, 50(7), 1036–1063.

Dark mode does have real uses: a dark room, light sensitivity, and some conditions where glare from a bright background is the main problem. The general picture is in dark mode and eye strain. With astigmatism, the balance moves further toward light mode.

What fixes it

1. Correct the astigmatism

This is the real fix, and the only one that works in every app. An eye exam measures astigmatism as a cylinder value with an axis. Glasses correct it directly. Ordinary soft contact lenses do not correct significant astigmatism; toric lenses do. If your prescription is more than two years old, or you got glasses for distance only, ask specifically about astigmatism at screen distance. Even a small uncorrected amount can be enough to make dark mode uncomfortable.

2. Use light mode when the room is lit

On a Mac: System Settings → Appearance → Light, or Auto to switch to dark only in the evening. In a bright room, light mode keeps your pupil narrow and puts the blur where you notice it least.

3. If you keep dark mode, lower the contrast

Pure white on pure black is the worst case. Off-white text on dark grey cuts the glow a lot. Most code editors and terminals offer lower-contrast dark themes. In Safari, Reader view has grey and sepia backgrounds. Avoid thin, light font weights; a regular or medium weight at a larger size smears less.

4. Add room light

A dark room with a dark screen opens your pupil as wide as it goes. A desk lamp that lights the wall behind the screen narrows it and also reduces the contrast between screen and room. This alone often makes dark mode tolerable.

5. At night, dim the screen instead

If you use dark mode at night only because light mode is too bright, lower the brightness and use Reduce White Point (System Settings → Accessibility → Display). You keep the sharper light-mode text without the glare. The other display settings that matter are in Mac display settings for eye strain.

The short version

• Glowing, smeared white text in dark mode is halation. Astigmatism is the usual cause.
• If squinting sharpens it, it is optical. Get an eye exam and ask about cylinder.
• Use light mode in a lit room. Use dark mode only in a dim one.
• In dark mode: off-white on dark grey, heavier and larger text, a lamp behind the screen.