https://pmc.ncbi.nlm.nih.gov/articles/PMC4340974/

Although diagnosis relies on observations made while patients are awake, the prevalence of sleep disturbances in individuals with ADHD is reported to be in the range 25–55 % (Corkum et al. 1998; Hodgkins et al. 2013; Owens 2005; Sung et al. 2008). In a recent Australian study, 62 % of children with ADHD had moderate or severe sleep problems and 22 % took sleep medications during the 1-week observation period (Efron et al. 2014). Indeed, high nocturnal activity and disordered sleep were defining characteristics of ‘hyperkinetic reaction in childhood’ or ‘attention deficit disorder’ in earlier versions of the DSM (American Psychiatric Association 1980; Barkley 1990; Sadeh et al. 2006; Spruyt and Gozal 2011).

My hot take is that *a ton* of ADHD cases is primarily lack of restorative sleep due to sleep disordered breathing, that is suffocating in sleep. Brain does not enjoy suffocating so people tend to wake up. Every couple minutes. Whole night. Without actually being aware that they do. They feel tired and brain fogged, lose concentration easily, have poor impulse control and so on. Even "mildly" disordered breathing in sleep causes frequent microarousals that fragment sleep and basically obliterate its restorative quality.