I have to admit that I do love my Kindle - it's so easy to slip into my bag to carry around with me. However at night I cherish the feel and scent of the paper in a traditional book.
Now researchers from Harvard Medical School have published a study showing that people who read from a backlit device in the evenings take longer to fall asleep than those who read from the printed page. They also spend less time in REM sleep and are groggier in the mornings. It's thought that short-wave enriched 'blue light' emitted by such devices suppresses (even more than other kinds of light) the production of melatonin, the hormone that controls the body's day/night patterns.