My money is doing nothing more than 80% of the time!

Since the stock market is only active from 9:30-4 M-F, (less than 20% of the hours in the week, and even less of the year because of holidays), is it possible to sell at the end of the day and plop the money into a savings account, and then buy again on the beginning of the next day? If not, what prevents this from happening? Is it just fees? Would be an easy way to make an extra 4%/year from interest.