DALLAS (CBSDFW.COM) – As users spend more time gazing into computer screens of all sizes during their waking hours, doctors say they’re seeing more patients complaining of what’s known as Computer Vision Syndrome.

Those who suffer from it complain of tired and blurry eyes, two of just a handful of symptoms associated with the syndrome.

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Dr. Michael Burton, a Vision Service Plan optometrist with First Eye Care in South Dallas has a laundry list of effects: “Eye fatigue, stress, blurring, redness of the eye, rubbing of the eye, insufficient focus,” he said.

“Just not being able to maintain clear vision consistently when you’re looking at these objects for periods of time.”

Patient Dariel Johnson complained to Burton of having tired and blurry eyes. She said she’s staring at a screen “probably from the time I wake up to the time I go to sleep.”

Johnson isn’t alone: According to a New York Times study, 31 percent of people older than 18 spend at least five hours a day on their smart phones, tablets and computers.

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Burton said the syndrome differs from book fatigue because of the handheld devices themselves.

“These devices have their own light sources,” he said. “And because they do, it puts a different source of stress on the eye.”

To avoid eyestrain on smart phones, tablets and computers, Burton recommends the 20/20/20 rule: “Every 20 minutes, look at a distance of 20 feet for 20 seconds,” he said.

“That gives an opportunity for the eye to rest, a break for the muscle focus and allows them to work at the task more efficiently and for a longer period of time,” he said.

Burton said the eye-breaks allow computer users to use the devices as long as they’d like without suffering any of the effects of Computer Vision Syndrome.

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Doctors recommend toddlers not use electronic devices and that older children use the devices for no more than two hours a day.