I may be wrong here. What I have seen is that the stock price of companies like AAPL has already changed before the opening bell of NASDAQ due to earning call result or a news from the Asian market. For example, AAPL had a bad quarter and the price dropped by $25 after hours. When the market opens at 8:30 AM EST, the price is already down. Setting a stop market or stop limit may not help because they don't get triggered after hours.
What is the best day and best time to buy stocks or options? What is the best day or best time to sell stocks? Is there a time in a day when the market is little stable to make such choice?