Day Trading VS Investing
There is a distinct difference between day trading and investing. The main difference is the time frame and methodology used. Investing requires a much longer time frame than trading, from months to years to decades. Usually you want to select a good company that will not go bankrupt the next day you purchase it. You will also want to analyze the fundamentals of the companies, make sure it is in good financial health and has a competitive advantage relative to other companies in the industry.
Trading takes a different approach to making money. The time frame considered is short from a few minutes to hours to days, weeks or maybe a month. Specifically, day trading refers to strictly trading within the day. This means that you do not hold positions overnight. For example, if you buy at 10:00 (EST), you have to sell before 16:15(EST) when the market closes.
There are no rules against holding overnight but risk is minimized if trading is strictly restricted to within the day. The market often moves in reaction to news when exchanges are closed. Stocks usually do not have much liquidity and trade on light volume after market hours. Imagine what would happen to your long position when there is a sudden hurricane strike when market is closed. The market will drop but you might not be able to sell at a reasonable price due to low volume. I sleep better at night when I know have no open positions overnight. Whatever losses and winnings are strictly during market hours when there is enough volume to trade. How the market moves after the closing bell does not affect me and I start the next day with a new state of mind.