Pulling up to their nearest grocery store or supermarket, shoppers will often see large windows spread across the front entrance. But once inside, natural light all but disappears. Stores rarely have windows along walls, near the front, or skylights. What gives?

The lack of windows is a component of retail shopping psychology, where retailers try to create an environment where people feel comfortable to spend both time and money. It’s why many stores have tile floors, which cause carts to make a lot of noise—forcing shoppers to instinctively slow down and browse more, or why stores may play up-tempo music when a store is busy to speed shoppers up.

In the case of windows, stores are after a sense of suspended time, where shoppers won’t notice inclement weather or encroaching darkness. The longer they shop, the more they spend.

There are practical considerations, too. Large windows letting in sunlight might cause fading on packages, making them seem worn to consumers. Swapping out valuable wall space for windows would also reduce the number of displays—and products—available to shoppers.

It’s possible this retail theory could eventually go out of fashion. Discount supermarket chain Aldi has experimented with stores in Germany that let in more natural light, though UV coatings are needed to prevent sunlight from affecting fresh food quality or product label fading.

Whether that will become more common in the grocery industry at large remains to be seen. If you’re feeling stifled by the lack of light, there’s always a farmer’s market.

[h/t Business Insider]