Having trouble hiring? No wonder your employees' hours are getting longer.

In a number of sectors where companies are struggling to find qualified workers, current employees have ended up putting in more hours, says a recent study by business research organization The Conference Board.

"As employers experience talent shortages, they are likely to shift the workload to existing workers," the report says. 

Co-authored by The Conference Board's director of macroeconomic research, Gad Levanon, and researcher Ben Cheng, the study compares data from the Bureau of Labor Statistics on the average weekly work hours from May 2011 to April 2012 to those of 2005-2007.

Across the board, the average number of hours worked per week declined slightly, slipping 1.5% from 39.7 to 39.1. But in several fields, workers significantly increased their number of hours worked.

Extraction workers--those who extract oil and gas--had the largest jump in the number of hours worked, with a 6% increase (51.1 to 54.2) over the time span. Second was communications equipment operators, whose weekly work increased by an hour, for a 3% jump. Math and science occupations also saw a 2.9% increase in weekly hours, and education, training, and library occupations rose 1.5% over the time span.