If you have ever looked at a chart and noticed an empty space between one day’s close and the next day’s open, you have seen a gap. A gap down open is when a share, index or futures contract starts trading well below where it finished the previous session. It usually means something important happened while the market was closed, and it can catch out traders who were not prepared.
What causes a gap down
Markets do not stop reacting just because the official session has ended. News keeps arriving overnight: company results released after the close, economic data from other time zones, political events, central bank comments or geopolitical shocks. When the market reopens, buyers and sellers immediately reprice to reflect that news. If the news is bad, the first trade can be far below the last close, leaving a visible gap on the chart.
Election nights are a classic example. Results arrive while London and New York are asleep, futures move in thin overnight trading, and cash markets open at very different levels the next morning. Our Morning Market Brief from the day after the 2026 US midterms shows how quickly that repricing can happen.
Why gaps are risky
The biggest danger with a gap is that a stop-loss order may not protect you as you expect. A stop is an instruction to sell if the price reaches a certain level. If a share closes at 100 and your stop is at 95, but it opens at 88, your order is triggered at the first available price, which may be close to 88 rather than 95. That difference is called slippage, and it can make losses bigger than planned. We cover this in more detail in our guide to Nasdaq futures gap risk.
Do gaps always fill?
You may hear traders say that “gaps always fill”, meaning the price eventually returns to the previous close. That is not reliable. Some gaps fill within hours as the first reaction fades. Others mark the start of a new trend and never fill, especially when the news genuinely changes the outlook for a company or the economy. Treating gap-filling as a rule is a common way to lose money.
How traders approach a gap down
Experienced traders tend to wait before acting. The first 15 to 30 minutes after a gap can be very volatile as orders from overnight are processed. Watching whether the price keeps falling or starts to recover gives more information than jumping in at the open. They also ask what caused the gap. A gap on company-specific bad news is different from a gap caused by a broad market move that may reverse.
Position sizing is the other key tool. Holding smaller positions over events such as elections, earnings or central bank decisions reduces the damage if the market gaps against you. For more on that, see our explainer on chasing overnight gaps into London.
The beginner’s takeaway
A gap down is the market catching up with news it could not react to while closed. It is not automatically a buying opportunity or a reason to panic. Understanding why it happened, and planning for the possibility before it does, is what keeps a gap from turning into a serious loss.
Handling gaps well is a real test of risk management. To find out where your own strengths and weaknesses lie, take the free Samuel & Co Trading Trader Assessment.
