Physicians are warning against looking at your smartphone while lying in bed at night.

Thursday, doctors, in the New England Journal of Medicine, detailed the cases of two women, ages 22 and 40, who experienced “transient smartphone blindness” for months.

The women complained that they had cyclical episodes of loss of vision for up to 15 minutes.

Despite medical exams, heart tests and MRIs, doctors couldn’t find anything to explain what the problem was.

An eye specialist, on the other hand, was able to figure out what was happening.

He learned that both women had a habit of looking at their smartphones with only one eye while in the dark.

Local optometrist Dr. James Barton said he hasn’t seen transient smartphone blindness in his practice, yet.

“Looking at your smartphone or any computer monitor really at night is strongly discouraged, even though I do it,” he said. “It messes with your sleep/wake cycle by lowering the secretion of melatonin. The blue light from the phones suppresses melatonin twice as much as green light.”

Barton said that suppressing melatonin changes circadian rhythm, which means that it takes longer to fall asleep, you have less REM sleep, and take longer to wake up the next morning.

“So, we encourage people not to read on an electronic device for two to three hours before bedtime,” he said. “If you must, dim the display and look into glasses that help block some of the blue light.”