Market Hours, Gaps & Circuit Breakers

The session clock

US equities: 9:30am–4:00pm ET, with pre-market from 4:00am and after-hours until 8:00pm (thinner liquidity both times). Overnight, global markets (Asia, Europe) keep repricing the world — which is why US stocks often GAP at the open instead of gliding there. The open is where overnight news gets priced violently; the last hour is where institutions finish their business (highest volume).

Circuit breakers

After 1987, exchanges installed circuit breakers: S&P 500 down 7% (Level 1) and 13% (Level 2) trigger 15-minute pauses; 20% (Level 3) closes the market for the day. Individual stocks have Limit-Up-Limit-Down bands that halt trading when price moves too far too fast. Halts are not crashes — they are pressure valves that let information get absorbed.

Practical implications

Never use market orders at 9:30:00 — spreads are widest and fills worst. Earnings release AFTER the close or BEFORE the open: the move happens in the gap, and your stop cannot protect you inside a halted or gapped print. Plan positions around event dates, not just price levels.

Sources

• Trading Hours & After-HoursInvestopedia
• Market Wide Circuit BreakersSEC.gov