We went for nearly a year working 7 and sometimes 8 shifts a week. A year with no days off. Tough schedule. When they finally got capacity up, we started getting Sunday nights off. We were ecstatic. With in a few weeks, our productivity and general morale dropped.

Why would this happen with a lighter schedule?

Your workplace examples might be an increasing or a decreasing schedule - but your HFE countermeasure is still needed.