Definition
US regular hours run 9:30 a.m. to 4:00 p.m. Eastern. Extended-hours sessions let some brokers trade before the open and after the close. Earnings and news are often released in these windows, so prices can move before the regular session reopens.
Watch out for
Volume is thin and spreads are wide in extended hours, so prices can be jumpy and may not carry into the next regular session. Many brokers only allow limit orders then.