Your iPad or smartphone could well be hurting your sleep patterns, even if you're getting in a full eight hours of shuteye, researchers say—and the effects could be long-term.

Over a five-night period, researchers in Boston had some subjects read for four hours a night on an iPad; others did their reading on printed books, with the lights low, the Washington Post reports.

Among members of the iPad group, levels of melatonin, a chemical behind sleepiness, decreased. It took them longer to drift off by an average of 10 minutes, the Wall Street Journal reports.

And once they did, their periods of rapid eye-movement sleep were shorter. The effects weren't limited to the nighttime. Their circadian rhythms were affected, and the next day—even if they'd slept eight hours—they reported being more tired.

And in the long term, reduced melatonin may boost the risk of various forms of cancer (prompting the rather frightening headline "Reading an iPad in Bed May Increase Cancer Risk," the Atlantic notes).

At issue is what's known as "blue light," the kind often given off by backlit gadgets, the Journal adds. "We introduce these devices that have medical and biological effects without requiring any health studies on their impact ... They don't have to go through any evaluation like a drug would, for safety and efficacy," says a researcher.

"I think it's time to rethink that." (As far as sleep goes, here's your "optimal" amount.)

This article originally appeared on Newser: Screens Mess With Our Sleep���Even If We Get 8 Hours

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