In our modern world, people spend hours on end staring at computer screens, smartphones, tablets, e-readers, and books that require their eyes to maintain close focus.

For most people (all except those who are nearsighted and aren’t wearing their glasses), their eyes’ natural focus point is far in the distance. In order to move that focus point from far to near, there is an eye muscle that needs to contract to allow the lens of the eye to change its shape and bring up-close objects into focus. This process is called accommodation.

When we accommodate to view close objects, that eye muscle has to maintain a level of contraction to keep focused on the near object. And that muscle eventually gets tired if we continuously stare at the near object. When it does, it may start to relax a bit and that can cause vision to intermittently blur because the lens shape changes back to its distance focal point and the near object becomes less clear.

Continuing to push the eyes to focus on near objects once the focus starts to blur will began to produce a tired or strained feeling in addition to the blur. This happens very frequently to people who spend long hours reading or looking at their device screens.

An additional problem that occurs when we stare at objects is that our eyes’ natural blink rate declines. The average person blinks about 10 times per minute (it varies significantly by individual) but when we are staring at something our blink rate drops by about 60% (4 times per minute on average). This causes the cornea (the front surface of the eye) to dry out faster. The cornea needs to stay moist in order to see clearly, otherwise little dry spots start appearing in the tear film and the view gets foggy. Think about your view through a dirty car windshield and how much that view improves when you turn the washers on.

So what should you do if your job, hobby, or passion requires you to stare at a close object all day?

Follow the 20-20-20 rule. Every 20 minutes, take 20 seconds and look 20 feet into the distance. This lets the eye muscle relax for 20 seconds, and that is generally enough for it to have enough energy to go back to staring up close for another 20 minutes with much less blurring and fatigue. It also will help if you blink slowly several times while you are doing this to help re-moisten the eye surface.

Don’t feel like you can give up those 20 seconds every 20 minutes? Well if you don’t, there is evidence that your overall productivity will decline as you start suffering from fatigue and blurring. So take the short break and the rest of your day will go much smoother.