Sometimes it does go down - I think the linked article says you need to be buying and selling billions of dollars of stocks for a long time to make the profit.

Sometimes it does go down - I think the linked article says you need to be buying and selling billions of dollars of stocks for a long time to make the profit. But another reason is that people are selling it before close, which lowers the price then buy again on open - which increases the price. Many day traders will close out their trades at the end of the day and start again the next day so they can't loose anything overnight, although they can't gain anything either - they're willing to loose that for the peace of mind.