A customer pays for her groceries after shopping at a Walmart store ahead of the Thanksgiving holiday in Chicago.

Walmart is reducing hours at its U.S. stores to allow more time to replenish shelves and clean and sanitize stores, the retailer announced Saturday in a letter on its website.

The national retailer said its stores will be open from 6 am to 11 pm until further notice. The new hours take effect Sunday.

The retailer has more than 4,700 Walmart and Neighborhood Market locations in the U.S. The shortened hours affect about 2,200 stores across the country that are open 24 hours a day.

Walmart is the largest grocer in the U.S. and has been dealing with a significant increase in sales as customers flock to stores to stock up on food, toilet paper, hand sanitizer and other items. Some grocers, including H-E-B, have adapted to the surge of shopping by limiting customers' purchases of food and cleaning supplies. Others like Kroger are advertising immediate job openings to keep up with heightened demand.

Stop & Shop, which is owned by Ahold Delhaize, also announced changes to store hours on Saturday. It said most stores will be open 7:30 am to 8 pm starting Monday. The company said it was suspending its online pickup service due to "unprecedented demand as well as product shortages in some areas."

Walmart has given its store managers the discretion to limit purchases of items that are in high demand.

In a letter posted on the retailer's website Saturday, Walmart's U.S. Chief Operating Officer Dacona Smith said the change "will help ensure associates are able to stock the products our customers are looking for and to perform cleaning and sanitizing." He said the retailer's "supply chain and trucking fleet will continue to move products and deliver to stores on their regular schedules."

Smith thanked Walmart employees for working hard to keep shelves stocked. Associates will continue to work their normal shifts despite the stores being open fewer hours.

"I don't think any of us have been through an experience like this, and we continue to be amazed at what our people, whether in the stores or in the supply chain, are doing to make sure customers have what they need," he wrote.