The original post doesn't mention IQ. IQ is a terrible metric for studying short-term cognitive effects.
Why wouldn't IQ tests be great for studying short term cognitive effects? I would trust their values for these type of studies the most: how does lack of sleep, being drunk, after exercise, etc affect your ability to solve abstract problems.
It has far fewer variables to account for than predicting someone's long term success.
IQ testing is designed to test for intelligent behaviour, which is largely based on long-term neurological development. The connections in the brain, densities of neurons, and long-term potentiations don't change greatly on a short-term basis. Measures which fluctuate based on spot-performance factors are typically deemed unreliable for the purpose of IQ testing.
Cognitive deficits caused by exposure to drugs and other substances tend to impact functioning of the neurons, and are detrimental to measures of intelligence, but are more appropriately measured by more specific tests, which focus on factors such as reaction time, or motor accuracy. Consider the cognitive tests performed by police when driving under the influence of alcohol is expected, for instance. Such cognitive tests also help provide a clearer picture of how a substance affects the brain.
In addition, to me, it sounds that you are arguing that a test on which you do worse when tired, sleepy, hungry etc. is not an appropriate test because it fluctuates depending on your state of mind. I don't get that.
I meant behaviour in the general psychological / cognitive sense; how an actor responds to stimuli.
Current popular IQ tests are designed to test many facets of intelligent behaviour -- verbal comprehension, arithmetic, working memory, symbol searching, etc. Abstract problem solving is only one category of testing.
I'm arguing that to study short-term cognitive deficits, it makes more sense to use other measures of cognition. It appears this is actually what was done, but this was incorrectly re-reported as "IQ".