Decision fatigue is a problem with exhaustion of cognitive endurance rather than a problem with cognitive overload. The former, like physical endurance exhaustion, occurs from becoming tired due to exercise over time without sufficient rest. The later occurs in a short time when the variables comprising a single, or focused set of, decision overwhelm the performance of a timely or adequate decision/response.
When a person becomes tired from continuous exercise, whether physical or mental, there are symptoms that occur that negatively impact performance. These symptoms are a way of telling you that the body is entering a state of heightened stress that can be resolved, to some degree, by appropriate rest. Stimulants work in the body to increase some performance factors while simultaneously impairing the association between exhaustion with the need for rest.
While these results from the use of stimulants can increase performance in a short term they are not a substitute for rest. Performance will degrade once the short-term high from the stimulant fades and especially as continued use results in tolerance to potentially catastrophic results.