As the human body exerts itself, it produces fatigue poisons, mostly as a byproduct of metabolizing glucose. These chemicals build up in the body, eventually causing a feeling of exhaustion. Larger amounts, however, can produce interesting side effects, which is why people who've stayed up late tend to get silly. Eventually the buildup becomes too great and the subject is forced to sleep so the body can focus on the removal of the poisons. Massage may also be helpful.

Interestingly enough, a Google search on the topic yielded quite a bit of information on various comic book characters. The significance of this is unknown.

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