2214 middle-aged British civil servants participated in the present study which was a part of the Whitehall II study. They took five different cognitive tests in 1997—99 and again in 2002—04. When compared with employees with normal working hours (35-40 per week), employees working very long hours (> 55 hours per week) and those with an average length of working week (41—55 hours) performed worse in a vocabulary test already at the baseline study. At follow-up, both over-time groups performed again worse in a vocabulary test and also had a declined test score in a cognitive reasoning test.
This, of course, was in the UK. Can you imagine the loss of cognitive skills among workers in countries without adequate overtime legislation and enforcement?