The BSE benchmark Sensex lost over 40 points in the opening trade today on emergence of profit booking at existing higher levels.The Sensex, which had gained 713.39 in the last four sessions, fell back by 40.27 points or 0.20 per cent to 19,952.43 levels in the first ten minutes of trading.Similarly, the wide-based National Stock Exchange Nifty index fell by 20.40 points to 5,991.30 points.