Why isn’t the NYSE open 24 hours? Why does it open at 9 a.m. and close at 4 p.m., only Monday through Friday?
Perhaps there was some good reason for it long ago, but in these days of instant info, electronic interconnection and people trading stocks without brokers from their kitchen tables or walking down the street with a Palm Pilot, why not have round-the-clock market activity in the U.S.?
All of the safeguards against widespread panic sell-off could still be in place, such as a trading shutdown for a “cool-off period” if the Dow dropped a certain number of points in a certain time period.
“Nothing is so firmly believed as what is least known” - Michel Gyquem de Montaigne
First, you get the car out of the garage. THEN you sit on the Long Island Expressway for 4-6 hours. THEN you “relax” in the Hamptons. THEN you drive BACK to NYC. That’s why the NYSE keeps banker’s hours. 
Cartooniverse
If you want to kiss the sky, you’d better learn how to kneel.
A lot of supermarkets aren’t open on a 24x7 basis, either. One of the ones in town routinely closes at 2200, and another at midnight (there’s a third one, but it’s so far out in the boonies that we never go there, except when they have a special on cut-to-order meat; I have no idea when it closes).
Trading hours are partly determined by tradition, just as prices in bits (12½ cents) have been. I believe that we’re looking at extended trading hours in 2Q2000, however (and that the NASDAQ, at least, is looking at pricing in decimals in 3Q; I’ll see if I can get more certain dates). There is also the convenience of having an acknowledged stop-point, where prices can be gotten, and valuations updated, without having to ensure that they’re from exactly 1730, or whatever, each day. Additionally, there is, as Cartooniverse suggests, the “wear and tear” factor, not only on the brokers and traders, but on the customers (wait until extended trading hours arrive, and everyone is hunched over their terminals at 2100).
Finally, until fairly recently, most shareholders were bankers, or the equivalent. If the customers are only going to work those hours, why shouldn’t the businesses that serve them?
“I don’t just want you to feel envy. I want you to suffer, I want you to bleed, I want you to die a little bit each day. And I want you to thank me for it.” – What “Let’s just be friends” really means
Perhaps the better question is: why do we need a physical stock market at all any more? Aren’t the vast majority of trades handled electronically anyway? Aren’t the trading floors becoming a bit anachronistic now?
I can understand why those who make their living on the market would not want to have watch their stocks 24x7, lest they be asleep when the crash hits… But if there’s big money in doing it, I wouldn’t rule out the possibility of continuous trading within the next few years.
The Pacific Exchange in San Francisco is going to be closing its floor and going online.
Obfus.
The classic argument against 24x7 market hours is that today’s stock markets are all basically auctions (with multiple participants bidding to purchase a given stock while multiple other participants are simultaneously bidding to sell it), and there will be far fewer participants during the off-hours. (Most/all of the corporate entities will have closed up for the night, for example, leaving just individuals and a few hard-core traders around during the wee hours of the morning.) That means that that we can expect much higher volatility in the markets during the off-hours, and a poor widow who just absolutely, positively has to sell her shares of AT&T at 3AM may well not get the best price for it.
Personally, I don’t see this as a problem. If/when the markets start displaying higher volatility during the off-hours, smart traders (or traders that write smart computer programs) will start trading during those off-hours to take advantage of that volatility - and end up reducing it because of their presence and activity. It’s a self-damping system.