20-20-20 rule timer.

The rule is one sentence: every 20 minutes, look at something about 20 feet away for 20 seconds. That is the whole technique. It is aimed at digital eye strain — the tired, dry, aching-behind-the-eyes feeling after a long stretch at a screen, which clinicians also call computer vision syndrome.

The reason it targets distance rather than screen time is that the eye is not straining because a screen is a screen. It is straining because you have held one short focal length for hours, which keeps the focusing muscle contracted, and because concentrating on a display roughly halves how often you blink, which dries the surface of the eye. Twenty seconds of far focus relaxes the first and a few deliberate blinks help the second.

Where it came from

The rule is usually credited to the American optometrist Jeffrey Anshel, who proposed it as a memorable version of advice that already existed about taking visual breaks. The three twenties are a mnemonic: they were chosen because they are easy to remember and hard to get wrong, not because a study identified 20 minutes as the point at which the eye is in trouble.

It stuck because the major eye-care bodies adopted it. The American Academy of Ophthalmology recommends the 20-20-20 rule in its guidance on eye strain and computer use, and optometric associations in several countries say much the same thing. That is the source of the advice on this page.

What the evidence does — and does not — support

Most pages about this rule say it is clinically proven. It is more accurate to say it is widely recommended and lightly evidenced. The trials that have tested the 20-20-20 rule specifically are few, small and short, and their results are mixed; there is no large body of work establishing that 20 minutes beats 30, or that 20 seconds beats 10. Several studies find that the harder problem is adherence — people dismiss the reminder and carry on working.

What is on firmer ground is the surrounding picture. Digital eye strain is real and common, blink rate does fall during screen work, and symptoms generally respond to breaks, to blinking, and to treating dryness. It is also worth knowing what the evidence does not show: screen use is not known to cause permanent damage to an adult eye, and blue light from screens has not been shown to damage it either, which the American Academy of Ophthalmology states plainly.

So the sensible reading is this. The 20-20-20 rule is free, takes twenty seconds, is recommended by the people who treat eyes for a living, and cannot hurt you. That is a good enough reason to try it. If your symptoms are persistent or painful, it is not a substitute for having your eyes examined — uncorrected long-sightedness, an out-of-date prescription and dry eye disease all present as screen fatigue.

How to use the timer above

Press start and go back to work. When the rising chime sounds, look up at the furthest thing you can see — out of a window is ideal — and let your eyes sit there for the twenty seconds the timer counts. Blink properly a few times while you are there. The falling chime means the break is over. You do not need to stop typing, save anything, or leave your chair; the break is for your eyes, not your task.

The timer keeps time from an absolute clock, so leaving this tab in the background cannot make it drift, and the remaining time is mirrored into the tab title. If you want a cue you cannot miss, keep the volume up rather than the tab visible.

Two things make the habit stick better than the timer alone. Move the screen to about an arm’s length away and slightly below eye level, so you are looking down at it and less of the eye surface is exposed. And take a real break away from the desk every couple of hours — the eye break is not a substitute for standing up.