In truth it is open all around the clock, but it depends on where you are based.
While trades can be negotiated practically at all times of the day, many investors and traders will prefer to do it when there are more active participants involved so as to get the best price. It is also less risky and spreads will not be as wide.

The stock market is not open 24/7 mainly because it relies on regulated exchanges, human oversight, and supporting institutions such as banks and clearing houses. Traditional markets like the NYSE and NASDAQ were built around business hours, allowing time for settlement, record-keeping, and maintenance. Limited hours also help reduce the risks of extreme volatility that might occur if trading never paused. While technology makes round-the-clock trading possible, markets still need breaks for updates, system checks, and to give investors time to process information. Some after-hours trading exists, but liquidity is lower and spreads are wider. Unlike cryptocurrencies, which run nonstop, stock markets follow structured schedules to maintain stability, efficiency, and fairness for all participants.