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Americans spend an average of 21 hours per week on their smartphones, but a new study reveals that this may have harmful effects.

Researchers wrote in The New England Journal of Medicine that excessive time on a device can result in health issues, including "transient smartphone blindness."

On "Fox and Friends Weekend" this morning, Kurt "The Cyber Guy" Knutsson explained that the study found that looking at a smartphone in the dark can lead to temporary blindness in one eye, which they dubbed "transient smartphone blindness."

Knutsson revealed that they studied two women who had similar symptoms of inhibited eyesight and couldn't figure out why.

The researchers determined that reading the smartphone in the dark with one eye - while the other is against a pillow or otherwise closed - leads to one eye becoming light-adapted and the other eye blocked becoming dark-adapted.

Knutsson said that, luckily, this appears to be a temporary condition, but the study found other health issues that could be caused by excessive smartphone use.

In particular, he said, there have been increasing numbers of young people with arthritic hands from texting and bad necks from holding their head at a angle to see their phones.

"Everything in life is about moderation," Knutsson said. "When you sense your body saying, 'Ouch, my hands are kind of hurting.' Well, guess what? Stop what you're doing and listen to your body. And you're likely to cure it on your own before it turns into a big problem."

Watch more above.

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