State securities regulators say they are concerned that extending trading hours will encourage investors to day trade.
The North American Securities Administrators Association, an umbrella group for state securities regulators, issued a statement Wednesday warning investors not to change their investment habits once the switch occurs.
The statement was issued to coincide with a joint summit on extended trading hosted by the Securities and Exchange Commission, the National Association of Securities Dealers Inc., and the New York Stock Exchange.
The trade group for state regulators has been particularly concerned with day trading in recent months. A task force plans to publish a report on the practice this month.
The "vast majority" of investors ought to be in the market for the long term and not take part in after-hours trading, a spokesman said.