First thing in the morning, market volumes and prices can go wild. The opening hours represent the window in which the market factors in all of the news releases since the previous closing bell, which contributes to price volatility. A skilled trader may be able to recognize the appropriate patterns and make a quick profit, but a less skilled trader could suffer serious losses as a result. So if you’re a novice, you may want to avoid trading during these volatile hours – or at least, within the first hour.
However, for seasoned day traders, that first 15 minutes following the opening bell is prime time, usually offering some of the biggest trades of the day on the initial trends. The whole 9:30 to 10:30 AM period is often one of the best hours of the day for day trading, offering the biggest moves in the shortest amount of time (a great and efficient combination). Extend it out to 11:30 AM EST if you want another hour of trading. A lot of professional day traders stop trading around then, as that is when volatility and volume tend to taper off. Once that happens, trades take longer and moves are smaller with less volume.