Friday at 4pm New York time, the S&P 500 cash session closes at 5,780. By Sunday evening the futures market is already quoting 5,750, thirty points lower, before most New York traders have poured coffee. Monday's open is not a continuation of Friday's chart. It is a decision the market made while the exchange was shut, priced in one jump instead of a thousand small ticks.
Gaps unsettle traders who think of price as continuous, because for long stretches of the week it is. Forex trades nearly around the clock from the Sunday evening open to the Friday evening close, and crypto never closes at all. But indices, oil and even forex carry real closed windows: the weekend, a daily settlement pause, a holiday. Whatever happens in the world during that window still gets priced, all at once, the moment trading resumes.
Where a gap actually comes from
A gap needs two things: a closed market and news that would have moved price if the market had been open. Nasdaq futures pause briefly around 5pm New York time and reopen at 6pm; anything released in that hour, an earnings surprise, a geopolitical headline, shows up as a jump on the reopen rather than a gradual move. The weekend is the biggest version of this. Two full days of headlines, data and positioning changes land on a single Sunday-evening reopen in forex, or a Monday 9:30 open in equities, compressed into one print instead of forty-eight hours of ticks.
Gold and oil behave the same way around their own daily settlement breaks, typically a short pause in the late afternoon New York time before the next session opens. The pause is brief, usually under an hour, but a surprise headline that lands inside it still prices in as a gap rather than a smooth move.
The gap-fill idea, and where it stops being reliable
Traders often repeat that gaps get filled, meaning price eventually trades back through the level it jumped away from. There is something to this: a gap frequently represents an overreaction or a level that was fair value only hours earlier, and price does return to test it more often than pure chance would suggest. What is not true is any fixed timeline or guaranteed outcome. Some gaps fill within the hour. Some take weeks. A meaningful number never fill at all, because the news behind them permanently changed what the instrument is worth. Trading a gap fill as a mechanical rule, betting every gap closes today, is a way to lose steadily to the minority of gaps that mean exactly what they say.
Handling positions and stops across a close
A stop loss only works while the market is open to fill it. Hold a EURUSD position over the weekend and a stop set at 1.0820 does nothing if Sunday's reopen prints at 1.0790: the fill happens at the new price, not your stop, and the difference is real, unplanned slippage. The same applies to indices and oil across their daily pause and any holiday closure. Traders who hold positions through a known closed window should size smaller than a normal intraday trade, precisely because the stop's protection is suspended for that window and the eventual fill could land well past it.
Trading the gap itself
On the reopen, two behaviors dominate. A gap-and-go continues in the direction of the jump, usually on genuinely new information, and tends to show early follow-through rather than hesitation. A gap fade reverses quickly, often on a smaller or purely technical gap with no real news behind it, back toward Friday's close. The safer approach waits for the first fifteen to thirty minutes of the new session to show which one is happening, rather than guessing at the open itself, when spreads are often at their widest and the fill you get is rarely the price you saw.