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 investors to buy or sell securities and, as a result, investors are more likely to pay or receive a competitive price for securities purchased or sold. There may be lower liquidity in extended hours trading as compared to regular market hours. As a result, your order may only be partially executed, if 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 executed 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 traded in extended hours trading may not reflect the prices either at the end of regular market hours, or upon the opening the next morning.