I can't recall the sources right now and I couldn't find a mini thesis (tesina in Spanish) that I wrote about the subject half a year ago. However, the research

I can't recall the sources right now and I couldn't find a mini thesis (tesina in Spanish) that I wrote about the subject half a year ago. However, the research started since the late 1800's - and if I remember correctly - from a German factory owner. They correlated working hours with productivity, finding that 40 hours a week is the optimal amount of working time for an employee. * There was an article on HN around June-August from a fella who did some research into it focusing it towards developers. * There is research on how sleep deprivation affects the brain. Those two may give you enough to look into. I'll owe you the links.