Using a smartphone in bed can trigger a temporary loss of vision, experts warn.
This has led to a rise in patients visiting hospitals complaining of problems in one eye.
Casualty doctors often mistake the symptoms for a transient ischaemic attack, or mini-stroke - a warning sign that a major stroke could be on the way.
But experts at Moorfields Eye Hospital in London say the symptoms, which last around 15 minutes, are usually the result of looking at a screen in bed.
Many people lie on their side at night, with one eye against the pillow and the other staring at the screen of a smartphone or tablet.
While one eye adjusts to the dark, the other gets used to the light of the device.
But when they roll over and open both eyes, the one that has been staring at the screen cannot cope with the darkness.
'With both eyes uncovered in the dark, the light-adapted eye is perceived to be "blind",' said the neurologists, writing in the New England Journal of Medicine.
They said that while the impact on sight is only temporary, investigating the problem as a potential stroke wastes NHS cash and causes anxiety to patients.