If you curl up with an e-book before bed every night, transfixed by its glowing light with the covers over your head, you might want to break this habit. A small study in Proceedings of the National Academies of Sciences says it might be inhibiting you from getting healthy sleep.

For the study, researchers had 12 participants either read from a paper book or a light-emitting e-book for four hours before bedtime for five days. For the next five days, the participants traded methods of reading.

The researchers then took blood samples to measure melatonin levels, and electronically tracked how long it took to fall asleep and how much time was spent in each sleep stage.

Compared to a printed book, a light-emitting e-book decreased sleepiness, REM sleep, and levels of melatonin, the hormone that usually rises before bedtime. In other words, e-book readers took longer to fall asleep and had a harder time waking up in the morning compared to print book readers.

But it's not only your iPad or eReader that's causing you sleep problems. Your phone, LED monitor, and laptop are doing the same thing. They all expose you to blue light, which is beneficial for the daytime because it tells your brain to stay alert, but detrimental for nighttime for the same reason.

"In the past 50 years, there has been a decline in average sleep duration and quality," said co-author Charles Czeisler, in a press release. "Since more people are choosing electronic devices for reading, communication and entertainment, particularly children and adolescents who already experience significant sleep loss, epidemiological research evaluating the long-term consequences of these devices on health and safety is urgently needed."

So while it's nice to be able to shut all the lights off before crawling into bed, you should probably keep 'em on if you like to read at night. And if you prefer the convenience of an e-book to the real page-turning of a printed book, choose an e-book that doesn't emit light. You may find out you're actually a morning person after all.

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