TOKYO -- The U.S. stock market is set to move toward nearly round-the-clock trading in early December, giving investors in Japan unprecedented flexibility to buy and sell shares in the world's largest equity market.

The prospect of longer trading hours is generating excitement among individual investors. Some institutional investors, however, are biding their time, weighing how the change might affect market liquidity and price formation.

Major U.S. stock exchanges plan to introduce 23-hour trading on Dec. 6, allowing investors to trade shares during daytime hours in Asia.

"The U.S. market is home to most of the leading artificial intelligence stocks attracting investment from around the world," said KEN, a 31-year-old investor who offers market commentary on YouTube and other platforms under that name. "If I can trade immediately after U.S. companies announce their earnings without having to wait for the market to open at night in Japan, I'll have more options," he said.

KEN began investing while still at university. Although he focuses mainly on Japanese equities, he also holds U.S. stocks. Convinced that AI will soon "permeate every aspect of daily life," he now allocates 40% to 50% of his portfolio to AI and semiconductor stocks.

He expects the longer trading hours to make U.S. shares more susceptible to fluctuations in Asian markets, a prospect that he sees as an opportunity. "If a stock I've been watching falls to a price I consider undervalued, I'd like to buy more during the extended trading hours," he said.

The NYSE Group and Nasdaq at present conduct regular trading sessions from 9:30 a.m. to 4 p.m., U.S. Eastern Time. Both also offer extended-hours trading, including pre-market trading from 4 a.m. to 9:30 a.m. and after-hours trading from 4 p.m. to 8 p.m., although schedules vary among exchanges within the NYSE Group.

Under the planned expansion of trading hours, participating exchanges will introduce a new overnight session from 9 p.m. to 4 a.m., extending trading to 23 hours a day, five days a week. Trading will pause for one hour, from 8 p.m. to 9 p.m., for system processing and maintenance. The trading week will begin at 9 p.m. Sunday and end at 8 p.m. Friday. The new overnight session will run from 11 a.m. to 6 p.m. Japan time, or from 10 a.m. to 5 p.m. when daylight saving time is in effect in the U.S.

Because the new hours partly overlap with the trading day in Tokyo, investors in Japan, South Korea and elsewhere in Asia will gain far easier access to U.S. stocks.

"It does seem like the vast amount of demand for 23-hour trading will come from the Asia Pacific region," said Chuck Mack, senior vice president for North American Markets at Nasdaq.

A Tokyo office worker in his 40s who invests in U.S. technology shares noted that until now, relatively few Japanese brokerages have offered after-hours trading in U.S. shares. He hopes the move to 23-hour trading will prompt more Japanese brokerages to expand their services and encourage greater trading activity. "If that happens, I'd like to trade individual U.S. stocks during the day in Japan, rather than limiting myself to exchange-traded funds, which can be traded outside regular U.S. market hours," he said.

But Tapazou, a popular blogger in his 50s who writes about U.S. equities under that name, cautioned against becoming too absorbed in the market. "Longer trading hours won't improve your returns if all you do is stare aimlessly at price charts and watch the market," he said. He urged investors to set clear goals and limit the time they spend monitoring stock prices, rather than allowing short-term market fluctuations to dictate their decisions.

Asset management firms, including those that operate mutual funds, are weighing how best to take advantage of the move to 23-hour trading. "If liquidity in U.S. stocks improves outside regular trading hours, we would like to consider trading individual U.S. stocks during the day in Japan," said Taro Iida, chief fund manager at Mitsubishi UFJ Asset Management, who oversees a fund that invests in U.S. technology stocks.

Iida's team begins work around 6 a.m. Japan time, gathering and analyzing information on U.S. stocks. After internal meetings, it places orders with brokers that evening for execution during regular U.S. trading hours.

The team instructs brokers to buy shares at prices determined using the volume-weighted average price (VWAP), a benchmark commonly used by institutional investors when placing large orders. VWAP measures a stock's average trading price over a given period, weighted by the number of shares traded at each price. This approach aims to reduce the risk of being unable to buy shares at the desired price if stock prices fluctuate sharply during regular U.S. trading hours.

For now, the team largely avoids after-hours trading because trading volumes are thin and bid-ask spreads tend to be wide. But if longer trading hours attract more participants and liquidity improves, Iida said, "We will also consider placing limit orders during the day in Japan, before regular U.S. trading begins and prices start moving." For institutional investors, however, longer trading hours are not necessarily an unalloyed good.

Tokio Marine Asset Management offers a U.S. growth-stock fund whose portfolio is managed by U.S.-based Voya Investment Management. Two of Voya's portfolio managers, Mark Phanitsiri and Moritz Dufner, cautioned that trading U.S. shares almost around the clock could increase short-term share price volatility following earnings announcements and other market-moving events.

Major institutional investors, such as life insurers and pension funds, which typically rebalance their portfolios every few months, are generally taking a wait-and-see approach.

Nippon Life Insurance does not expect to change how it executes trades to rebalance its portfolio, on the assumption that liquidity in U.S. stocks will remain limited during daytime hours in Japan. Sompo Holdings and MS&AD Insurance Group Holdings acknowledged that the longer trading day will create more opportunities to execute trades. But both said the extent to which they use the expanded hours will depend on market liquidity and transaction costs relative to those seen during regular U.S. trading hours. More trading opportunities, they noted, will not necessarily translate to more trades.

Japan's Government Pension Investment Fund (GPIF), one of the world's largest institutional investors, oversees some 300 trillion yen ($1.9 trillion) in assets. It currently has no plans to change its investment management practices in response to the longer trading hours. However, GPIF believes the extension will give its external asset managers greater flexibility in deciding when to buy and sell U.S. shares in the cash market.

While investors differ in their enthusiasm for the change, the push toward nearly round-the-clock trading is spreading beyond the U.S.

In July, the London Stock Exchange announced plans to launch London Stock Exchange 24 (LSE 24), a new venue designed to provide near-continuous trading from Monday through Friday. The new venue will operate separately from the exchange's main market, complementing its existing trading hours. Buying and selling of exchange-traded products, including exchange-traded funds, is expected to begin in the first half of 2027, subject to regulatory approval. The exchange eventually plans to expand the range of eligible securities to include individual stocks.

Julia Hoggett, CEO of LSE and head of digital and securities markets at LSEG, described the planned launch as "an important step in the evolution of our markets," saying it would help create a more digital and interconnected global marketplace.

In Asia, South Korea has also extended its stock trading hours. On Sept. 14, the Korea Exchange (KRX) introduced an after-hours session from 4 p.m. to 8 p.m., replacing a more limited system that operated until 6 p.m. The new session allows continuous trading in most stocks listed on the KOSPI and KOSDAQ markets, although exchange-traded funds, exchange-traded notes and certain other securities have been excluded for now.

Extending trading hours, however, does not automatically translate into more active markets. Brokerages that handle and route orders, along with other market participants, face additional costs for staffing and system upgrades.

Clearing and settlement procedures, as well as handling of shareholder records, also require coordination across the financial industry. Whether the benefits of longer trading hours will outweigh the additional costs remains uncertain.

"We will continue to study the issue, taking into account the views of market participants and developments in overseas markets, but we currently have no plans to extend trading hours," said Hiromi Yamaji, group CEO of Japan Exchange Group (JPX).

Instead, the Tokyo Stock Exchange is focusing on a different strategy to bolster its appeal to investors and companies. It is seeking out promising startups across Asia, helping them expand into Japan and forge partnerships with Japanese businesses, with the ultimate goal of encouraging them to list their shares in Tokyo.