Many Walmart locations are open 24 hours. The store has traditionally kept most of its stores open around the clock, but as of 2015, it is closing increasing numbers of stores at night in certain markets.
Walmart is currently evaluating the profitability of 24-hour stores in each of their markets and reducing hours at some that it feels are underperforming. If there is a strong demand in an area that Walmart serves for 24-hour shopping, it likely has at least one store that is always open. Just about all of Walmart's "Supercenter" locations are open 24 hours, but they usually do not staff the deli or bakery overnight.