Basic Points

• After 4pm EST, the cash session ends, and so does trading on most options. But options on some key products are available until 4:15pm EST, including SPX, SPY, QQQ, IWM, and DIA. 
• The normal hours (known as the cash session) are between 9:30am and 4pm EST time. Depending on the broker, stocks can be traded before and after this period for a certain amount of time, but not options on those stocks. 
• The premarket can open as early as 4am EST depending on the broker. And the postmarket can be open as late as 8pm EST. 

Intermediate: Liquidity and After-Hours Risks

There is much more liquidity and volume during the cash session. Consequently, prices can move much faster and in a wider range after hours when this volume is missing. 

The bid/ask spreads are typically wider outside normal hours as well. Only the most liquid stocks and ETFs can maintain tight bid/ask spreads outside regular hours. SPY, for example, usually manages to keep its bid/ask spread down to a penny regardless of what time it is in the premarket or postmarket. But the spreads will widen enough on other stocks or ETFs to the point that they essentially become untradeable. This means that the bid/ask spread should always be checked and double checked when trading at these off-hours. Otherwise, a trade could fill at an unfair price as compared to normal hours.

Orders typically need a special flag or checkmark for “Outside RTH” (Regular Trading Hours). Additionally, GTC (Good to Close order), or a similar time in force parameter, is needed outside of regular trading hours for trades to execute. 

The after-hours segment is divided into three parts. First, there is the postmarket. Depending on the broker, the end of this may vary between 6pm and 8pm EST time. Afterward, there is the period in between the end of the postmarket and the beginning of the premarket, which we can call the overnight session; the significance of this overnight segment is that futures can be traded but stocks and ETFs [at most brokers] cannot. And then, also depending on the broker, the premarket might start anywhere between 4am and 9:30am EST time.

Areas of time outside regular trading hours have an important distinction from the cash session. One major difference is that options on stocks are untradable at these off hours. Also, normal stop orders, which are based on market orders, do not work either at these times. However, stop limits will work outside regular trading hours and so will options on futures. Another set of functioning instruments during most of the premarket and postmarket are major index products like SPX or NDX.