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Two women in the UK went temporarily blind in one eye, and their doctors think using their cell phones in bed was the cause. Here's how you can avoid the same problem.

You don't need a doctor or an online health expert to tell you that you should unplug from your electronic devices when you go to bed.

Constant access to social media, instant messaging, email, and website updates gives the digitally overconnected a kind of technological hangover. Several years ago researchers at Michigan State University published the not-especially-surprising conclusions of a study that found that people who were on their phones for work after 9 p.m. were less productive and more fatigued than their unplugged counterparts. Smartphones are "almost perfectly designed to disrupt sleep," one of the authors of the study, Dr. Russell Johnson, told the press. Because smartphones keep us mentally connected to work until late in the evening, they make it harder to fall asleep and stay asleep.

Even worse, light emitted by cell phones prevents the pineal gland of the brain from making the hormone melatonin to help you get to sleep. Visible blue light in the frequencies between about 420 and 500 nanometers stops the production of the sleep hormone. Before there were cell phones, computers, televisions, and night lights, this ability to detect the first hints of morning light kept people from oversleeping in the morning. When the eyes are constantly bathed in blue light, however, it keeps people from getting to sleep in the evening.

Just how detrimental is the presence of blue light? Our eyes are so sensitive to blue light that it registers on our retinas even when our eyes are shut. Having a light on won't completely prevent sleep for most people, but it takes, on average 10 minutes to fall asleep in an environment in which blue light is detectable. Once people get to sleep, having a light on in the room can deprive them up to 50 minutes of rapid eye-movement (REM) or dream sleep, leaving them notably groggier the next morning.

But it isn't just feeling groggy that's the problem. When the same people get up, their vision is fuzzier. The reason for this is a phenomenon known as over-accommodation. In the very middle of the retina there is a small indentation called the fovea. This part of the retina has unusually sharp vision. However, it doesn't have a lot of the cones that detect blue light. The retina is more sensitive to blue light in a ring around the fovea known as the parafovea. 

When you operate a smartphone in the dark, you basically have a tiny blue light emitter in bed with you. The blue light from the phone stimulates the parafovea, where you have fuzzier vision, but not the fovea, where you have sharper vision. As a result, when you get up the next morning in normal light you have a fuzzy view of the world around you. You can't see distant objects as well as you normally can. If you are middle aged or older, you may not see anything at all clearly for a while after you get up. In extreme cases, there can even be blindness.

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