Shoppers at the 'Toys R Us' store in Doral, Fla.

U.S. consumer sentiment fell more than expected in the December mid-month reading, slipping further below the decade high reached in October.

The University of Michigan's survey of consumer attitudes for December declined to 95.9, after rising to 98.5 in November. Economists polled by Reuters expected the December reading to slip to 97.1.

The indicator has remained largely unchanged in 2017, which the survey's chief economist, Richard Curtin, has said reflects American consumers' increasing confidence about their income and employment prospects.

"Consumer confidence continued to slowly sink in December, with most of the decline among lower income households," Curtin said in a statement.

The measure soared to 101.1 on Oct. 13 — the highest level since 2004 — but has deflated steadily since.

The index measures 500 consumers' attitudes on future economic prospects, in areas such as personal finances, inflation, unemployment, government policies and interest rates.