Since it is used to judge one's abilities, it's hardly fair to use it as a practical measurement, just as BMI is impractical to use for judging a body builder's fat percentage.
Anecdotally, medication which I take for ADHD/PI doesn't permanently alter my brain's chemistry, yet my IQ takes a noticeable dive without it.
Of course bad sleep will affect your IQ, and it doesn't make it less reliable. Why? You brain is less reliable when you had bad sleep, and IQ is a measurement of its peak performance. If you're not at peak performance any given day, your results will show.
No one who does well on an IQ test likes to be told an IQ test means very little, keeping in mind that high IQ doesn't determine your success, or guarantee you any specific lifestyle.
IQ is more of a horoscope than it is a method of science.
Being able to reach your peak IQ potential consistently is part of the daily hygiene of eating well, sleeping well, exercising well. And it's also a question of making the effort. If I can't measure it, it means you don't reach it. Or at least not often enough.
On the other hand, I agree that it might be overstated. But having worked with people that don't have the best minds, you start to believe IQ is not just crap.
If I use a measuring tape to record the length of a piece of wood - unless I cut the piece of wood, or damage it in some way - this measurement will always be the same. In the cold, it might shrink and in the heat it may expand - but I can measure those variations and account for them using the basic laws of math. Can you do the same with IQ?
> But having worked with people that don't have the best minds, you start to believe IQ is not just crap.
It's not that I don't believe intelligence varies, I just don't think we have the ability to measure it - yet.
What about those taxi drivers who had enhanced brain regions for geographic knowledge? Kind of the same thing...