Wal-Mart Stores Inc. will start closing some of its 24-hour supercenters for at least a few hours each night, aiming to use the time to better stock shelves and organize stores for the peak shopping rush. The move will affect about 40 stores, including those in Philadelphia, New Jersey and Maryland, beginning next week, said spokesman Brian Nick. Chief Executive Officer Doug McMillon is taking the step as part of his drive to improve the shopping experience at Wal-Mart’s U.S. stores, where customers have complained about empty shelves, long checkout lines and poor-quality produce.