When I got my iPad first gen back in 2010 there was an issue with syncing to iTunes and my screen stayed on all night. I posted in these forums asking the same thing...isn't it harmful to leave the screen on this long. Most answered back saying the type of screen it is that prolonged use shouldn't degrade the screen over time.
I had no issues with my screen from then on, no ghosting or degredation in quality over time.