Devices like the Kindle, the Nook (the top part of the screen that displays books) and popular e-readers from Sony use a technology called e-paper. It simulates the look of an actual printed page and does not emit light. That means, unlike the iPad, you can effectively read in direct sunlight. (Beach, anyone?)
The iPad, however, contains a touchscreen liquid-crystal display that, like computer screens and television sets, emits light. On the plus side, you can sneak the device under the covers while your significant other sleeps beside you and flip through a couple pages of a book without a flashlight.
But staring at the screen before bed could leave you lying awake. That’s because direct exposure to such abnormal light sources inhibits the body’s secretion of melatonin, say several sleep experts.