Use It Too Much and Lose It? The Effect of Working Hours on Cognitive Ability [pdf]
I didn't read it in detail, so forgive me if this was accounted for,  but I feel like this would be hard to control as people who are not working at all or working extremely long hours may not be as capable as those working a normal or slightly lower than normal numbers of hours to begin with.For example, if I can get a job and make enough money in a shorter number of hours, I'm probably smarter than the person who can't get or a job or the person who has to work 65+ hours a week, even if accounting for degree or similar.