An article in The New England Journal of Medicine recounts the cases of two patients both of whom presented with extended bouts of temporary blindness in one eye. The first was a 22-year-old woman who had been experiencing blindness in one eye for several months at night. The second was a 40-year-old woman who was experiencing 15 minutes of blindness in one eye each morning.

Doctors did the usual tests in such cases and found nothing to be wrong medically with the women, so they started asking more questions and found a common link: smartphone use in bed.

The temporary blindness was caused through the combination of looking at a very bright screen while lying on a pillow meaning one of the eyes is covered/blocked by a pillow. In this situation the eye able to see the screen becomes light adjusted to that level of brightness. When the blocked eye then is able to see again it renders the light adjusted eye blind and there’s an extended period of time where it remains blind while the other eye catches up and normal vision becomes possible again.

So, what’s the lesson from these cases? Always view your screen with both eyes, and I’d add that screen brightness should be lowered considerably when viewing in a dark environment. Either that, or turn a bedside light on to add more light to the environment and lower the strain on your eyes. But just as importantly, the general level of lighting in the room isn’t the major cause of this temporary blindness, it’s the fact one eye is not able to see the screen at all.