# Eye comfort: strain, flicker and blue light

> Screens do not damage the eyes, but long sessions tire them, and the causes are well identified: we blink less, we focus at a fixed short distance, and we often work against glare or with a badly adjusted screen.

Source: https://monitorselector.com/en/eye-strain-flicker-blue-light/ (Monitor Selector, 2026-11-14)

## In short

- The main cause of tired eyes is dryness: in front of a screen the blink rate falls by half or more.
- Every 20 minutes, look 6 metres away for 20 seconds: the focusing muscles relax and blinking resumes.
- Set the screen to the brightness of its surroundings, and keep windows to the side, never behind or in front.
- Blue light from screens does not harm the retina, and blue-filtering glasses do not reduce eye strain. Warm the screen in the evening for the sake of sleep.

## What digital eye strain is

Burning or dry eyes, blurred vision at the end of the day, difficulty refocusing from the screen to the distance, headaches around the forehead: this set of symptoms is known as digital eye strain or computer vision syndrome. Surveys find it in a majority of people who spend more than a few hours a day at a screen. It is uncomfortable, it is temporary, and no study has shown that it leads to lasting damage.

Three mechanisms explain most of it.

**Blinking.** At rest we blink about fifteen times a minute, and each blink spreads a fresh film of tears on the cornea. Concentrated on a screen, we blink a third to half as often, and many of those blinks are incomplete. The tear film breaks up, the surface of the eye dries: this is the origin of the burning and of the intermittent blur that a blink clears.

**Sustained focus.** To see clearly at 60 cm, the ciliary muscle contracts to curve the lens and the eyes converge. Held for hours without a pause, the effort ends in fatigue and in a slowness to relax, the blurred distance vision of the evening.

**Bad conditions.** Reflections on the screen, a window in the field of view, a panel much brighter or darker than the room, text too small or too far: each forces the visual system to compensate.

## Habits that work

**The 20-20-20 rule.** Every 20 minutes, look at something at least 20 feet away (6 m) for 20 seconds. The ciliary muscle relaxes, the eyes blink, the tear film re-forms. The numbers have nothing magical; the principle is to interrupt near focus regularly, and a glance out of the window does it perfectly.

**Blink on purpose** when you notice the burning, a few slow and complete blinks. In dry, heated or air-conditioned rooms, a humidifier or lubricating eye drops help; so does keeping the screen low, which narrows the opening of the eyelids and slows evaporation (see [viewing distance and posture](https://monitorselector.com/en/monitor-viewing-distance/)).

**Have your eyesight checked.** A small uncorrected defect, a slight astigmatism or the beginnings of presbyopia, goes unnoticed in daily life and becomes a constant effort at the screen. From the mid-forties, glasses computed for the distance of the monitor often remove fatigue that no screen setting could.

## Brightness and glare

The eye adapts to the average brightness of what it looks at. A screen far brighter than its surroundings dazzles; far darker, it forces the pupil to open and reduces sharpness. The rule is that the screen should have **about the brightness of a sheet of white paper** lit by the room: hold one up next to a white document on the screen and adjust until neither stands out.

In figures, that is 100 to 150 nits in an ordinary office, 200 to 300 near a window in daylight, 60 to 80 in the evening by lamplight. Most monitors leave the factory at 250 nits or more, too bright for a room lit artificially. An ambient light sensor, when the monitor has one, makes the adjustment by itself.

Glare is treated at the source:

- place the desk so that windows are **to the side**, the screen perpendicular to them. A window behind the screen dazzles; a window behind you is reflected in it;

- avoid ceiling lights directly above or behind your head;

- prefer a matte screen in a bright room;

- never work in the dark with a bright screen: a lamp behind or beside the monitor reduces the contrast between screen and wall.

## Flicker and PWM

There are two ways to dim a backlight. The first lowers the current in the diodes, which shine less, continuously. The second, pulse-width modulation (PWM), switches them on and off hundreds or thousands of times a second and varies the duration of the pulses: the eye averages, and sees a dimmer light.

***Fig. 1.** Two ways of dimming a backlight to 40%. The average is the same; only one of them flickers.*

Above a few hundred hertz nobody sees the flicker directly. But a low PWM frequency, around 200 to 500 Hz, combined with deep modulation, produces visible artefacts when the eyes move (a trail of distinct images behind a bright object), and a minority of people report headaches and eye fatigue with such screens. Sensitivity varies greatly from one person to another, and controlled studies are few.

The prudent approach costs nothing: most current monitors dim by direct current and are sold as **flicker-free**, a claim that independent tests verify. OLED panels are a special case: their brightness dips briefly at each refresh, a shallow modulation at the refresh rate that most people do not perceive. If you know you are sensitive, check the measurements of the model before buying.

## Blue light: what the evidence says

Two different questions are often confused.

**Does the blue light of screens harm the eyes?** Nothing indicates that it does. The damage shown in laboratories comes from exposures far more intense than a screen delivers; a monitor emits a small fraction of the blue light received outdoors on an overcast day. The American Academy of Ophthalmology does not recommend any special eyewear for computer use. As for blue-filtering glasses, a Cochrane review of 17 randomised trials concluded in 2023 that they probably make no difference to eye strain compared with ordinary lenses.

**Does it affect sleep?** Here the answer is yes, to a degree. Light rich in blue wavelengths in the evening delays the secretion of melatonin and shifts the body clock; intensity and duration matter as much as colour. The night modes of operating systems (Night Light, Night Shift), which warm the image after sunset, go in the right direction, and lowering the brightness goes further. For sleep, the most effective measure remains to stop using screens in the last hour of the evening.

“Low blue light” certifications on monitors therefore answer a concern that the evidence does not support for the eyes. They do no harm; they are not a reason to choose one screen over another.

## Text: size, sharpness, contrast

Text that is too small is read by leaning forward and squinting. At your distance, capital letters should be at least 3 to 4 mm high; if they are not, increase the [scaling](https://monitorselector.com/en/resolution-and-scaling/) rather than moving closer. Sharp characters also demand less focusing effort than blurred ones: a higher [pixel density](https://monitorselector.com/en/pixel-density-ppi/) is a real factor of comfort for those who read all day.

Dark text on a light background remains the most legible combination in a lit room: the bright background narrows the pupil, which increases the depth of focus. Dark mode is pleasant in dim surroundings and saves energy on OLED; in full light, and for readers with astigmatism, light letters on black tend to bleed. Use whichever you read more comfortably; neither protects the eyes.

## The checklist

- Screen at 50 to 100 cm, top edge at eye level or below, tilted back.

- Brightness matched to a sheet of paper lit by the room.

- Windows to the side; no bright source in front of or behind you.

- Capital letters 3 to 4 mm high at least.

- A look into the distance every 20 minutes.

- Warmer and dimmer screen in the evening.

- An eye examination if discomfort persists.

**Sources.** A. L. Sheppard and J. S. Wolffsohn, “Digital eye strain: prevalence, measurement and amelioration”, BMJ Open Ophthalmology, 2018. S. Singh et al., “Blue-light filtering spectacle lenses for visual performance, sleep, and macular health in adults”, Cochrane Database of Systematic Reviews, 2023. American Academy of Ophthalmology, “Should you be worried about blue light?”.

## Frequently asked questions

### Do blue light glasses help with eye strain?

Probably not. A 2023 Cochrane review of 17 randomised trials found that blue-light filtering lenses probably make no difference to eye strain from computer use. Dryness and sustained near focus are the real causes, and breaks, blinking and a well-adjusted screen address them.

### Is a flicker-free monitor important?

It is a sensible precaution that costs nothing, since most current monitors are flicker-free. Low-frequency PWM dimming bothers a minority of sensitive people; if you have had headaches with a previous screen, check independent flicker measurements before buying.

### What brightness should a monitor be set to?

About the brightness of a white sheet of paper lit by the room: 100 to 150 nits in an ordinary office, more near a window in daylight, 60 to 80 in the evening. Most monitors are set far too bright out of the box.