WASHINGTON — Black Friday shopping is shifting from hours spent in line to more time online.

Sales at retail stores on Black Friday fell 10 percent to $10.4 billion this year, down from $11.6 billion in 2014, according to research firm ShopperTrak. And sales on Thanksgiving dropped by the same percentage, to $1.8 billion.

A big reason for the decline is increased online shopping, as Americans hunt down deals on their smartphones, tablets and computers. And many retailers are offering bargains long before Thanksgiving, limiting the impact of Black Friday specials.

Online retailers have been bombarding customers with email discounts and bargains for weeks. Online sales jumped 14.3 percent on Friday compared with last year, according to Adobe, which tracked activity on 4,500 retail websites. Email promotions drove 25 percent more sales compared with 2014, the company said.

Brick-and-mortar retailers saw fewer customer visits on Thanksgiving and Black Friday, compared with last year, according to Kevin Kearns, ShopperTrak’s chief revenue officer.

“Shoppers are researching products ahead of time, targeting their store visits, and arriving in-store with the intention of making a purchase,” Kearns said.

The drop in Thanksgiving Day visits may also reflect a “social backlash” against stores opening on that day, Kearns said.

Chris Christopher, director of consumer economics at consulting firm IHS, said many retailers’ warehouses and store shelves were overstocked heading into the fall. That prompted many to offer deep discounts as early as the beginning of this month.

“The price discounting has been creeping toward Halloween,” he said.