Generally, the more orders available in the market, the more liquid that market is. Due to limited trading activity in the extended-hours trading sessions, the liquidity in these sessions may be significantly less than during regular market hours. Lower liquidity may prevent your order from being executed in whole or in part, or from receiving as favorable a price as you might receive during regular trading hours. In addition, lower liquidity means fewer shares of a given security are being traded, which may result in larger spreads between bid and ask prices and volatile swings in stock prices. Risk of Trading Halts.
News stories can have a strong impact on stock prices during extended-hours trading sessions.
The SEC, the NASD, or one of the stock exchanges may impose a trading halt when significant news has affected a company's stock price. An order entered prior to a trading halt will become eligible for execution once the trading halt is removed. However, any order entered after a trading halt has been declared will be rejected and must be reentered once trading resumes. Risk of Duplicate Orders. There is a risk of duplicate orders if you place an order in an extended-hours session for a security for which you already have an outstanding order in the regular trading session, even if that order is a day order.