In my freshman year of college, I ran into a grouping of circular dependencies that were so intertwined it would have warranted a total rewrite. I spent 28 hours fixing it. I took over my dorms common area, had about 5 movies playing on repeat for background noise, and stayed in the same seat for the whole period.
It was rewarding, but also showed me how easily I can get tunnel vision on a task. Now that I've graduated and moved on to a real job, I have certain back ups to make sure I don't forget to eat or take my breaks. I almost got in trouble for not taking them when I first started.