At my previous work, we attempted to find the correlation between faulty code and the time of commit in the version control. Most bugs were written after 14:00. (And no: we didn't check-in all code at the end of the day).
A fellow student of mine did a study back in 2005 about this. He found that the worst time to fix a bug was Friday afternoon, as it introduces the most (new) bugs into the code.
I see similarities to another area where I have more experience: business meetings. I believe that most people are terrible at meetings held in the afternoon, either due to heavy lunch, or just because it's after 3pm and they are simply tired. A single meeting like this can cause great damage to the individuals and whole teams, measured in intangibles such as motivation, trust, momentum.
