I haven't read the article but my OB said as long as im comfortable with it (which i was until a week ago!) then its not a major issue, but not to do it all the time. as PP said, its to do with bubs weight pressing down on a major vein (Vena Cava its called) which can lead to the dizzyness/fainting. I find now if i do lie on my back i get short of breath and am dizzy when i get up, so no more back sleeping for me i can't wait to sleep on my tummy again!