In almost every case, she says, the parent interrupted an activity with the child to use a device.

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There's also the mystery of just what is happening in a child's brain while using the device.

WSJ: What Happens When Toddlers Zone Out With an iPad

Interactive Story Books allow parents to make a video recording of themselves reading to the child using Zoodles.com or the device's built-in camera.

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Depending on his or her age, your child may use a hand-held device to measure how well he or she can breathe (peak flow meter).

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The main aim of the device, says Swain, is to combat child obesity.

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Each child was given an actigraph, a tiny wristwatch-sized device which allowed continuous monitoring without interfering with sleep.

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