After-hours trading can be a risky environment that is ill suited to many investors.
Too a large extent the after-hours market place is a news driven market. In many instances there are fewer active participants to react to and digest the news being released from public companies and other sources.
With fewer investors participating in this marketplace, securities have less "liquidity," which, among other things, can mean larger spreads between bid and ask prices. Investors who are unfamiliar with the workings of limit orders find themselves handicapped in the ECN marketplace. Additionally, depending on your brokerage firm and how they have implemented their after-hours trading session, the market centers that your orders can interact with may not include all possible execution venues. As a result your order may not be executed at the most favorable price available amongst all the market participants. There is also exists the possibility that your order may not get executed at all.
Since large institutions often play a bigger role in moving stock prices up and down in the thinly traded ECN environment, investors can find themselves whipsawed by even more severe price volatility in the after-hours market than is the case during regular exchange hours.
In short, the after-hours trading market is no place for amateurs!