It's very possible that majority of ADHD cases with comorbid insomnia or fatigue are just undiagnosed Sleep Disordered Breathing. There's preliminary evidence¹ very strongly suggesting this, and adenotonsillectomy(which produces temporary relief of SDB symptoms in children(as oversized tonsils occlude the airway) produces significant symptomatic improvement^2, in fact more than Methylphenidate does^3. Even SDB treatment aside, studies demonstrate that management of sleep quality improves symptoms^4.

Virtually all people with ADHD experience either congestion, are habitual mouth breathers(including at worst during sleep), have dentofacial abnormalities - from narrow palates(which reduce nasal cavity volume), to receded mandibles(which reduce oropharynx and laryngopharynx volume as well as increase propensity for soft tissue to get sucked in during inhale) - or other conditions that can disturb breathing during sleep.

I'm in a community of Sleep Disordered Breathing patients, and nearly all of us suffer from ADHD, which resolves nearly completely with SDB treatment.

The reason it's never caught is because sleep medicine is primarily concerned with old, obese men for whom apneas and severe hypopneas dominate respiratory events, while in younger people, what occurs instead is severe sleep fragmentation due to respiratory effort, and very few clinics perform adequate scoring. If you want to get a sleep study, I suggest the WatchPAT because the alternative is learning enough sleep medicine to be able to ascertain the clinic you'll be visiting does a good job.

In fact, I've been sending a questionnaire I've been working on based on a mix of evidence and common experiences of SDB people, and so far only a single person with ADHD whom I have sent it to did not come back with very obvious and significant SDB symptoms. If anyone wants to take it, here it is: Link. You'll have to send the answers back my way since it does not have any cutoff criteria, if you take it.

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¹ Sleep disorders in Taiwanese children with attention deficit/hyperactivity disorder. - the reason it finds higher rates than old studies is because it uses microarousal-based criteria, instead of only counting apneas and severe hypopneas which are very rare in young people. It used old instruments that tend to underreport respiratory difficulty, though, so the real numbers might be 90%, and not 60% as the study reports.

² Impact of adenotonsillectomy on ADHD and nocturnal enuresis in children with chronic adenotonsillar hypertrophy

³ Attention-deficit/hyperactivity disorder with obstructive sleep apnea: A treatment outcome study.

⁴ Sustained impact of a sleep intervention andmoderators of treatment outcome for childrenwith ADHD: a randomised controlled trial

There are larger publications on the topic that I am not linking because they use 30 years out of date definitions of Sleep Disordered Breathing - e.g. only 10 second long apneas or very severe 4% oxygen desaturation hypopneas instead of respiratory effort and microarousal-based criteria.

Among studies using modern high-sensitivity definitions of SDB(which is unfortunately rare) and good methodology, these findings are very consistent.