From December this year, the US stock market will move closer to nearly round-the-clock trading. Nasdaq, NYSE Arca under the New York Stock Exchange, 24X National Exchange, and Cboe EDGX under the Chicago Board Options Exchange have all laid out plans to add nighttime trading sessions: in addition to regular trading hours and the existing pre-market and after-hours sessions, a new session from 9 PM to 4 AM New York time will be introduced.
This expansion plan is expected to be implemented on December 6, with the aim of meeting growing trading demand from overseas investors while also responding to competition from cryptocurrency and prediction markets.
In fact, overnight trading in US stocks has existed for many years, mainly conducted through alternative trading systems (ATS), but it still accounts for a very small proportion of overall trading volume. In the second quarter of this year, overnight trading accounted for only about 1% of total stock trading volume.
However, despite being small in scale compared with daytime trading, overnight trading is growing rapidly, with a year-on-year increase of as much as 358%. The upcoming "23 hours, 5 days a week" trading model has also sparked debate: what impact will it bring to global market participants?
Supporters argue that extending trading hours can break down time zone barriers and make it more convenient for overseas investors to access the US stock market. Skeptics worry that lower liquidity and wider bid-ask spreads during overnight trading could instead amplify trading risks.
One analyst said that what institutional investors are mainly concerned about at present is "market quality after extended trading hours," while operational and staffing issues are actually not their biggest concern.
A survey last year showed that market participants at the time were not enthusiastic about round-the-clock stock trading. Because there are fewer participants outside regular trading hours, investors may face lower trading volume, thinner liquidity, and wider bid-ask spreads.
The survey also showed that this could add extra operational risk and complexity, and may even affect traders' own physical and mental health. However, market attitudes have improved somewhat since the survey was conducted in the second half of 2025.
In fact, the concept of overnight trading has been welcomed by retail investors and trading platforms for many years. Some analysts have also pointed out that institutional investors have no interest in trading in a market that is low in liquidity, wide in bid-ask spreads, and very likely highly volatile.
At present, pre-market and after-hours trading volume combined accounts for only about 10% of overall trading activity. In reality, some market participants have already pointed out that current market trading hours are already too long, rather than too short.
Currently, institutional investors generally remain on the sidelines. Overseas investors are naturally one of the most active groups in overnight trading. According to SEC data, in the second quarter of this year, overseas investors accounted for 37% of overnight trading volume in US stocks, while institutional accounts accounted for only 7%.
At the same time, overnight trading is highly concentrated in a small number of stocks. In August this year, on average only 15 stocks were needed to contribute half of all overnight trading volume. By comparison, during regular trading hours, 256 stocks are needed to contribute half of market trading volume.
Currently, Nasdaq and the New York Stock Exchange trade five days a week, with core trading hours from 9:30 AM to 4 PM US Eastern Time. In addition, they also provide pre-market and after-hours trading, with pre-market trading starting as early as 4 AM and after-hours trading lasting from 4 PM to 8 PM, though the types of trades that can be executed are relatively limited.
After the trading hours expansion in December, exchanges will still maintain a one-hour break each day between 8 PM and 9 PM for system maintenance and trade processing.