The index's survey found that 29% of U.S. workers believe the economy is getting stronger, down two percentage points from the April survey.

The number of workers confident in the future of their current employers fell to 63% in May from 68% in April. However, the number of workers confident in their ability to find a new job rose to 43% in May from 42% in April. And the number of workers saying there are fewer jobs available fell to 59% from 60% in April.

"Our latest report is still very positive despite a slight dip in overall confidence," said Roy Krause, president and CEO of SFN Group. "Not only are workers more confident in the number of jobs available, but over 40 percent are optimistic that they would be able to find one. Another positive sign that we are seeing is the increasing need for administrative and clerical workers. This shows that hiring has expanded beyond the light industrial sector -- which tends to lead in a recovery -- and investments are being made in front-office talent to support business growth."

The index's survey questioned 1,466 employed adults in the U.S.