Generally, the more orders that are available in a market, the greater the liquidity. Liquidity is important because with greater liquidity it is easier for the investors to buy or sell securities, and as a result, more likely to pay or receive a competitive price for the securities purchased or sold. There may be lower liquidity in the extended hours trading as compared to regular market hours. As a result, your order may only be partially executed, or not at all. Risk of Higher Volatility.
Volatility refers to the changes in price that securities undergo when trading.
Generally, the higher the volatility of a security, the greater its price swings. There may be greater volatility in extended hours trading than in regular market hours. As a result, your order may only be partially executed, or not at all, or you may receive an inferior price in extended hours trading than you would during regular market hours. Risk of Changing Prices. The prices of securities trade in extended hours trading may not reflect the prices either at the end of regular market hours, or upon the opening the next morning.