Extended-hours sessions run on the lit exchanges' electronic order books outside of the regular 9:30am-4:00pm ET window. Pre-market begins as early as 4:00am ET at some brokers; after-hours runs to 8:00pm. Unlike the overnight 24-hour session, extended hours uses the same SIP/UTP venues that handle regular-session trading, so price discovery is closer to the daytime session — but volumes are still a small fraction of regular-session liquidity.
Most retail brokers require explicit opt-in to trade extended hours and accept limit orders only. Market orders, stop orders and stop-limit orders are generally disallowed or auto-converted to limits. Exchange rules also restrict short-sale execution outside regular hours.
Earnings releases and macro data (CPI, FOMC) routinely trigger 5-15% moves in extended hours that do not survive the next regular open. The wider spreads and thinner books mean a market-impact-aware approach (small size, marketable limits, never chase) is essential.
