Stocks closed sharply lower after being down all day on Fed Chairman Ben Bernanke's remarks that "economic uncertainty" has increased.

"The market has these little flips up and down but it's been quiet and down all day long," said Mike Driscoll, head of listed trading at Bear Stearns. "It points to very extreme skittishness both ways here, and I don't think people have a handle on which way the market is going."