I've just posted this elsewhere, but my pain management consultant told me on several occasions that he didn't "believe in" ADHD. My son's impulsive behaviour often involved me trying to stop him running off or other things that were a danger to him when he was much younger. Tough when you suffer debilitating pain. So, that was Not Helpful.
An FYI on ADHD (happy for corrections if some of the below has since been updated) - it doesn't mean someone - child or adult - can't sleep. Although it can involve that (my son's official diagnosis is "ADHD with sleep difficulties").
Sleep difficulties can mean not needing much sleep. Or, it can mean that the mechanism for the child to be able to "switch off" and go to sleep doesn't work properly. Or, it can be both. With my son, it is the inability to switch off, for which he takes melatonin - the natural hormone his body doesn't produce very efficiently, to help him .
ADHD is a very heritable condition, which means that it often runs in families (see below regarding undiagnosed adults with mental health disorder diagnoses).
ADHD means there can be difficulty with impulse control. But equally, it can mean that there is also an issue with hyper-focusing (that's the medical term too btw) where the person/child can get deeply absorbed in something they enjoy, and be unable to be drawn away from it, causing issues with "transition times".
ADHD can display behaviours that can be construed as ASD type behaviours, that, upon managing the ADHD, disappear. However, a child can have both types of disorder at once to varying degrees.
Medication sometimes works well to manage ADHD symptoms. The medication provided is a stimulant (not a form of sedative or dampening of behaviour) which stimulates the area of the brain that isn't functioning properly to allow a person to maintain attention. Caffeine can have a similar, albeit milder, effect on the brain as ADHD meds, and so it could be said (in my personal opinion) that people who "need coffee" may indeed be self-medicating......
ADHD affects boys and girls almost equally (with girls more likely to have ADD rather than ADHD), but often each displays differently between the sexes. Girls, like with ASDs, are woefully under-diagnosed because they are able to mask or imitate more, or self-manage more efficiently and seem to be 'functioning' okay.
Girls in particular (but there is considerable risk to both sexes), who are un-diagnosed and have no support in place, go on to develop anxiety, depression, bpd and other more "mainstream" mental health disorders as a result. Which is why a mother of a child who is deemed to be "just behaving badly" can have a history of attempted suicide and depression and both of them have a diagnosis of ADHD/ADD.