If the risk of an accident is essentially doubled at twelve hours compared to eight, what about the risk of inserting bugs? By the time a measurable accident occurs, the worker has probably been fumbling around, working erratically, for quite some time, and finally got so far off track that an accident was the result. My educated guess would be that the insertion rate for defects increases far more rapidly than the risk of industrial accidents. What the accident statistics probably tell us is that a tired programmer has double the chances of putting in a very serious bug, but has probably also put in a much higher percentage of smaller ones.
A recent Circadian study addressed productivity in white collar workers. They found that even as little as a ten percent increase in work hours could result in a 2.4 percent drop in productivity, while sixty hour weeks could result in a 25 percent drop in productivity.