In the US the 8 hour work day was established by FDR with one feature being that it would tighten the labor market (and reduce unemployment) by forcing employers to hire more people.
The lower hours also reduced workplace injuries and provided longer term economic gains [1]. This study is probably too short measure the arguably more important gains that would take a long time to show up.
1: The Rise and Fall of American Growth, by Robert Gordon.
1: The Rise and Fall of American Growth, by Robert Gordon.