Using smartphones in the dark while lying in bed at night can damage vision or cause temporary blindness.

Two women suffered temporary blindness for months due to regularly using their smartphones in the dark. The condition is called "transient smartphone blindness."

In a study published in The New England Journal of Medicine on June 23, doctors wrote about the incidents where two women, aged 22 and 40, complained about their conditions.

These women experienced recurring episodes where they temporarily lost their visions for up to 15 minutes each.

After conducting several tests including heart exams and MRI scans, the doctors failed to find what was causing the temporary loss of vision. It was only when they talked to an eye specialist that the mystery was solved.

According to Dr. Gordon Plant from the Moorfields Eye Hospital in London, he asked the women what they were doing when the episodes happened.

The women were reported to be looking regularly at their smartphones in the dark. While many people do this, the women checked their devices with only one eye while lying in bed on their sides with a pillow covering the other eye.

"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," said Plant.

When the women stopped using their smartphones, they found that they couldn't see using the eye they used to check their devices.

Plant explained that it takes "many minutes" for the phone eye to catch up with the resting eye that has already adjusted to the dark.

There's a simple solution: when using smartphones in the dark, use both eyes. Plant added that temporary blindness induced by this particular smartphone use is ultimately harmless. Cases like these are also easy to avoid.

One of the women felt relieved that the temporary blindness wasn't linked to a more severe health problem such as an impending stroke.

However, the second woman was not too quick to believe Plant's diagnosis. For a whole month, she kept a journal and tracked her episodes of temporary vision loss. Eventually, she believed Plant's explanation.

"People frequently use smartphones while lying down, when one eye can be inadvertently covered. Smartphones are now used nearly around the clock, and manufacturers are producing screens with increased brightness," wrote the researchers.

They also said cases like these can become more common, adding that the two cases suggest knowledge in retinal physiology and taking detailed notes can give assurance to both patients and doctors.

These can also help patients avoid anxiety and added expenses. The team included researchers from the Moorfields Eye Hospital, the National Hospital for Neurology and Neurosurgery, King's College and City University.