Here's a factoid for you. In January, workers in the manufacturing sector worked an average of 41.9 hours per week. That same figure was reached for a few months during the winter of 1997-1998, but the last time it was actually higher than that was July of 1945 when America's factories were working overtime to fight and win World War II. From January 1942 thrugh to the summer of '45, manufacturing hours were consistently at 42 or above but outside the context of total war it just never gets that high.
We're at the point, in other words, where just to sustain current levels of output American factories are going to need to start adding workers.