What happens to our brain as it becomes more and more fatigued, lets say in a non stop 12h study session? I know it decreases its productivity, but in what does this productivity decrease translates? Is it an overall slowing (i.e., you can still solve all the complex problems that you did when you weren't fatigued but at a slower speed) or is it also a decrease in the maximum limit of complex reasoning one can achieve (i.e., you can spend all the time you want trying to solve a problem but you just won't be able to solve it without resting)? What mechanisms are responsible for this (synaptic failure, etc.)? Are there any articles related to this issue?