LONDON — Warning: Looking at your smart phone while lying in bed at night could wreak havoc on your vision.

Two women went temporarily blind from constantly checking their phones in the dark, say doctors who are now alerting others to the unusual phenomenon.

The solution: Make sure to use both eyes when looking at your smart phone screen in the dark.

In Thursday’s New England Journal of Medicine, doctors detailed the cases of the two women, ages 22 and 40, who experienced “transient smart phone blindness” for months.

The women complained of recurring episodes of temporary vision loss for up to 15 minutes. They were subjected to variety of medical exams, MRI scans and heart tests. Yet doctors couldn’t find anything wrong with them to explain the problem.

But minutes after walking into an eye specialist’s office, the mystery was solved.

“I simply asked them, ‘What exactly were you doing when this happened?”’ recalled Dr. Gordon Plant of Moorfield’s Eye Hospital in London.

He explained that both women typically looked at their smartphones with only one eye while resting on their side in bed in the dark — their other eye was covered by the pillow.

“So you have one eye adapted to the light because it’s looking at the phone and the other eye is adapted to the dark,” he said.

When they put their phone down, they couldn’t see with the phone eye. That’s because “it’s taking many minutes to catch up to the other eye that’s adapted to the dark,” Plant said.

He said the temporary blindness was ultimately harmless, and easily avoidable, if people stuck to looking at their smartphones with both eyes.