Overnight risk is the chance that a position moves against you while your market is closed, your orders are inactive, or trading conditions make an exit difficult. A price gap can turn a planned loss into a much larger one before you have an opportunity to act.
For swing trading, the answer is not automatically to close every position before the bell. It is to separate ordinary price risk from event risk, then decide how much exposure you can carry through an overnight session, weekend or earnings announcement.
Why prices gap between trading sessions
An overnight gap is the difference between a market’s previous closing price and its next opening price. New information can change what buyers will pay and sellers will accept. The next session does not have to resume at yesterday’s valuation.
A company might issue a profit warning after the close. By morning, buyers may only be willing to trade well below the previous price. There need not be an opportunity to sell at each price between those levels.
Opening arrangements also matter. Exchanges can bring orders together in an opening auction rather than simply restart continuous trading. The NYSE opening auction procedures allow orders to accumulate before a security opens, and an individual security can open after the scheduled start of the session.
For planning purposes, distinguish a routine overnight hold from a known event. A position carried through a results announcement needs a separate assessment from one held during an otherwise uneventful week. Neither is risk free, but treating them as identical hides the decision you are making.
A stop order is not a guaranteed loss limit
A standard stop order becomes a market order when triggered. It does not reserve an execution price. If a share opens below your sell stop, the eventual sale can occur below that level. This distinction is central to FINRA’s warning about stop orders in volatile markets.
A stop limit order makes a different trade-off. It controls the acceptable execution price, but the order may remain unfilled if the market moves beyond its limit. Avoiding a poor sale price can leave you holding a falling position.
Consider this hypothetical trade in a £20,000 account. You buy 500 shares at £10 and place a sell stop at £9.60. The planned price loss is £200, or 1% of the account, assuming execution at the stop price.
| Actual sale price | Loss per share | Total loss | Account loss |
|---|---|---|---|
| £9.60 | £0.40 | £200 | 1% |
| £9.00 | £1.00 | £500 | 2.5% |
| £8.80 | £1.20 | £600 | 3% |
These are assumed fills, not predictions. An opening quote is not necessarily the price at which your entire order will execute.
The example also shows why moving a stop closer does not solve gap risk. You can reduce the distance between entry and trigger without reducing the number of shares exposed to an overnight repricing. Keep your swing trading exit rules, but do not treat the stop distance as a hard ceiling on loss.
What changes when you hold over a weekend?
Where a market closes for the weekend, you face a longer period in which news can arrive without an opportunity to trade that instrument. Elections, geopolitical developments and company announcements can change the assessment you made on Friday.
Event risk can also produce abrupt moves that an average daily volatility measure does not represent well. CME Group’s analysis of event driven price gaps illustrates the difference between ordinary fluctuations and occasional large jumps.
That does not mean every weekend deserves the same response. Assess what could happen before your next executable session, not just whether the calendar says Friday. A scheduled election and a routine weekend should prompt different questions.
Review positions together, too. As a stress test, suppose three £5,000 share positions each fall 8% following the same adverse development. Their combined price loss would be £1,200. Three separate trade tickets would not make that exposure independent.
Check the actual reopening time, holiday arrangements and your broker’s access for each instrument. Use the guide to UK market hours and international sessions when planning across exchanges. Do not assume that another market being open gives you an executable price for the position you hold.
Earnings announcements deserve a separate holding decision
An earnings release can change the basis of a trade. Instead of trading a pullback or breakout, you may be betting on results, management’s outlook and the market’s interpretation of both.
A company can beat earnings estimates and still see its shares fall if its outlook disappoints. Management commentary during the accompanying call can also add information beyond the headline figures. These are among the reactions covered in FINRA’s explanation of earnings announcements and share prices. “Good results” and “a profitable trade” are not interchangeable.
Before holding through results, verify the release date and timing against the company’s investor relations announcement. Record whether the date is confirmed or estimated. Check the time zone and the timing of the earnings call separately; do not assume the release and discussion happen together.
Then ask what your strategy actually requires. If the original setup was a short price swing before results, holding through the announcement changes the trade. It should require a fresh decision, not happen because selling feels inconvenient.
A useful written rule might be: “Do not carry an ordinary technical setup through scheduled results unless it qualifies under a separately tested event strategy.” That is an example of a rule to evaluate, not a universal prescription.
Also review upcoming announcements from businesses closely connected to your holding. Ask how you would respond if a major customer or competitor published news that challenged your original reasoning. The aim is not to forecast every announcement, but to identify exposures you might otherwise overlook.
Extended hours access does not remove overnight risk
Being able to trade outside the regular session offers another opportunity to act, but it does not guarantee a practical exit. There may be fewer buyers and sellers, wider spreads and greater price uncertainty. An extended hours price may also differ from the next regular session’s opening price. These risks appear in the SEC investor bulletin on extended hours trading.
Check whether your instrument is available, which order types are accepted and whether existing orders carry into that session. Do not assume that an order marked “good until cancelled” operates during every session your broker offers.
Make this check before an announcement, not while trying to exit afterwards. Keep a record of the relevant order settings and cancellation rules.
If your plan depends on selling immediately after a release, test whether that assumption is realistic for your account. Access is useful; it is not the same thing as dependable liquidity.
Size the position for an adverse gap scenario
Use a gap scenario alongside the ordinary stop calculation. Rather than asking only what happens if the stop executes as planned, ask what happens if the first practical exit is much worse.
Return to the £10 share example. Suppose you choose £8.80 as an adverse exit scenario and want the associated price loss to stay within £200. The calculation is:
Scenario share quantity = cash loss budget ÷ assumed loss per share
That gives £200 ÷ £1.20 = 166 whole shares, rounded down. At the assumed exit price, the loss would be £199.20 before costs. The original 500 shares would lose £600.
This is a stress test, not a guaranteed maximum. A lower exit price would produce a larger loss. The £8.80 assumption is illustrative, not a recommended estimate for any particular share.
When choosing scenarios, examine previous event reactions and test outcomes worse than the examples in your sample. Leave room for transaction costs rather than allocating the entire cash budget to the price move.
For an existing holding, assess potential giveback from its current value as well as loss from entry. Suppose the shares were bought at £8 and now trade at £10. A fall to £8.80 would leave a gain from entry, but it would still erase £1.20 per share of current account value. Calling it “house money” does not change the arithmetic.
Decide whether to hold, reduce or close
Hold when carrying the position through the event is part of the strategy you are evaluating, and the adverse scenarios fit your risk budget. Write down why the expected opportunity justifies remaining exposed. A strong opinion about direction is not a substitute for that calculation.
Reduce when you want to retain some participation but the full position creates an unacceptable scenario loss. In the earlier example, cutting 500 shares to 250 would halve the price loss at any identical assumed exit price. It would also halve participation in a favourable move. That trade-off is the point.
Close when the event falls outside the strategy, the potential loss is unacceptable, or your plan depends on an exit you cannot reliably execute. Missing a favourable gap is the cost of avoiding that exposure, not proof that the decision was wrong.
Put these conditions in your written trading plan before the decision becomes urgent. Rules should be clear enough that you can apply them to both winning and losing positions.
A profitable position should not receive an automatic exemption. Nor should a losing position stay open simply because closing it would make the loss feel final.
Run a short check before the close
Use a brief review rather than improvising at the end of the session:
- Events: Check announcements due before your next opportunity to trade.
- Access: Confirm session availability, order eligibility and holiday closures.
- Exposure: Calculate adverse scenarios for each position and the account together.
- Decision: Record whether you will hold, reduce or close, and why.
- Next action: Define what you will do if the market opens beyond your planned exit.
Set a review deadline that leaves enough time to act. A sensible decision made after your intended execution window has passed is still too late.
After a gap, review execution rather than rewriting the plan
If the market opens against you, check the position and order status before submitting another instruction. Establish whether an existing order has filled, partially filled or remains active. Avoid creating a second trade because you assumed the first instruction had failed.
Do not make “wait until it gets back to yesterday’s close” the default response. Reassess the position using the information and prices now available, alongside the contingency you wrote before the event.
For your journal, record the previous close, opening price, actual fills, costs, event and planned risk. Separate ordinary overnight holds, weekends and earnings trades so that one group does not hide the results of another.
Apply the same discipline to backtesting and strategy validation. A simulation should not assume a fill at the stop price when its own price data shows the market opening beyond it. State the execution assumption and test less favourable alternatives.
The practical aim is not to predict every gap. It is to avoid carrying a position whose acceptable risk depends on the market giving you an exit price it has never promised.