Sleep professionals are aware of the detrimental effects of smartphones in bed, but now there have been cases of “transient smartphone ‘blindness,'” reports NPR.

“They were looking at their smartphones and they just happened to have one eye covered because they were lying in bed,” says Omar Mahroo, an ophthalmologist at Moorfields Eye Hospital in London and an author on the letter.

The first patient had a habit of gazing at her smartphone before falling asleep. She’d lie on her left side and look at the screen primarily with her right eye. Her left eye was often covered by the pillow.