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.