>How much do you think ADHD is a specific mental condition that one either has or doesn't have, versus a set of common symptoms that can have many causes?
The sign has been pretty specific that it's the first (specific condition), and not the latter.
To get the big thing out of the way: brain scans of people with ADHD show measurable differences in brain structure, activity, and neurotransmitter levels compared with neurotypical brains[1].
Secondly, ADHD is highly hereditary, to the tune of 75%+ chance[2].
But you don't need a brain scan for your answer. There's a reason ADHD is a thing in DSM (if it were merely "a set of common symptoms", it wouldn't be).
Each "symptom" (a manifestation of an ADHD trait) in and of itself may not be rare, and people in general may experience it during certain conditions.
ADHD is experiencing a hella lot of them off the list, for absolute majority of the time, all your life.
Please really, really pay attention to the example I gave. It looks like you glossed over it entirely.
And yes, I'm fully aware of the irony of asking someone to pay more attention in this thread.
> I wonder which direction a causal relationship between sleep and ADHD might be.
The ADHD --> horrible sleep causation is well established.
The horrible sleep --> ADHD symptoms become worse causation is also well established.
Horrible sleep --> ADHD causation is nonexistent. There is nothing indicating that ADHD could be an acquired condition. There's all the evidence to the contrary (you are born this way, as the song goes).
The study establishes that "fixing" circadian rhythm helps with ADHD symptoms, which is a Captain Obvious moment (full night's sleep makes brain work better, and we have difficulty getting it ever).
I'm not sure what other causation you are looking for.
[1] https://www.adhdevidence.org/blog/finding-order-in-the-compl...
[2] https://pmc.ncbi.nlm.nih.gov/articles/PMC6477889/