Some of the biggest share price moves of the year happen when the main stock market is closed. A company reports results at 4.05pm in New York, and within seconds its shares are 8 per cent higher or 10 per cent lower. Nobody waited for the next morning. That is after-hours trading, and if you follow US shares from the UK, it is worth understanding how it works before you react to it.
What after-hours trading actually is
The regular US stock market session runs from 9.30am to 4pm New York time, which is usually 2.30pm to 9pm in the UK. Outside those hours, shares can still change hands on electronic trading networks. The session after the close is called after-hours or extended-hours trading, and it typically runs until 8pm New York time. There is a matching pre-market session in the morning before the opening bell.
These sessions exist because news does not keep office hours. Companies deliberately release quarterly results after the close so that investors have time to digest them, and big economic or political headlines can land at any time. After-hours trading lets investors respond before the next regular session.
Why earnings season makes it matter
Most large US companies report results either just before the open or just after the close. When a household name reports after the bell, the after-hours price becomes the market’s first verdict on the numbers. A strong quarter with a confident outlook can send the shares sharply higher; a disappointing forecast can do the opposite, even if the results themselves were fine.
Those moves often spill over. Futures on indices such as the Nasdaq 100 trade almost around the clock, so a big after-hours reaction in a heavyweight stock can lift or drag the whole index overnight. That in turn shapes how Asian and European markets open the next morning, which is why our guide to how futures bridge Asia into London is a useful companion to this one.
The catch: thin trading
After-hours prices are real, but they are not the same as prices during the main session. Far fewer buyers and sellers are active, so liquidity is thin. That has three practical consequences.
First, spreads are wider. The gap between the price you can buy at and the price you can sell at is often several times larger than during the day, which makes every trade more expensive.
Second, prices can swing more violently. With fewer orders in the book, a single large trade can move the price a long way. Early after-hours moves on results can overshoot and partly reverse once more information arrives, for example during the company’s conference call.
Third, order types are limited. Many brokers only accept limit orders in extended hours, because a market order in a thin market could be filled at a very poor price.
Reading an after-hours move sensibly
A useful habit is to treat the after-hours price as a first draft rather than a final answer. Look at why the shares moved: was it the headline profit, the revenue figure or the outlook for the next quarter? Forecasts usually matter more than the quarter just gone. Then watch whether the move holds into the pre-market session and the next day’s open, when liquidity returns.
It also helps to know how big a move the options market was expecting before the results. If options implied a 7 per cent swing and the shares move 5 per cent, the reaction is roughly in line with expectations, however dramatic it looks on a chart. Our beginner’s guide to reading an earnings report covers which numbers tend to drive these reactions.
What UK traders should keep in mind
Because of the time difference, US results often arrive in the UK evening and the after-hours session runs into late night. That makes it tempting to trade tired, on a phone, in a thin market. Many experienced traders prefer to let the dust settle and plan for the next session instead. If you hold US shares, it is also worth checking whether your broker offers extended-hours dealing at all, and on what terms.
The bottom line
After-hours trading is where much of the market’s first reaction to company news happens, and it often sets the tone for the next day across global markets. But it is a thinner, more expensive and more volatile place to trade. Understanding that is the difference between reading the move and being whipsawed by it. If you want an honest view of how you handle fast-moving news and where your process could be tighter, the free Samuel & Co Trading Trader Assessment is a sensible next step.
