Staring at your smartphone for too long can cause eye strain.
Scientists found just two hours a day looking at the screen may cause vision problems.
Eye strain, or visual fatigue, has long been a problem for office workers who spend too long in front of computers.
But increased use of smartphones means people are looking at screens for hours in their leisure time too.
Researchers recruited 65 people aged 18 to 30, almost 90 per cent of who preferred reading on devices to books.
They found more than half, 54 per cent, frequently or always suffered at least one problem linked to screens.The list of 17 ailments included dry and irritated eyes, sensitivity to light and headaches.
Those who looked at smartphones for at least two hours a day were likely to report red eyes, with those who spent up to five hours most likely.
People on their computer or laptop for at least one hour a day including reading, studying or streaming TV reported difficulty focusing their sight when they looked up across a longer distance.
Lead author Madison Frazier, from Arizona State University in the US, said: "Our study found there was an increased risk of people suffering visual fatigue, the longer they stared at screens.
"Our advice based on these findings is if people cannot put their phones or laptops away, because they need them, they could at least try to walk away from them for a couple of minutes every hour.
"Or they can turn down the brightness of the screen or keep it at arm's length," said Frazier.
The study, presented at the conference of the American Association for the Advancement of Science in Seattle, was set up to look at eye strain.
Frazier added: "Visual fatigue may not seem like a big problem, but it can cause headaches and a decrease in performance at school or work. Screens can create glare when the brightness is too high, or lighting bounces off the screen into the eyes."
Some studies show if people take more and longer breaks, this could help with these symptoms.
In 2018, researchers at Aston University in Birmingham warned of a rise in dry eye disease among children. It suggested it was due to too much time looking at screens so did not blink enough and developed burning, gritty eyes.