Being calm and relaxed on stimulants can potentially mean there is some arousal disorder: sleep apnea, narcolepsy, delayed sleep syndrome, or just chronic sleep deficit. Maybe concussions (mild TBI), amphetamines work for that too. Even depression is possible.
ADHD is just most likely because it’s so common, with sleep apnea being close second. I would rule out ADHD and sleep disorders first.
Potentially also, UARS, which is also form of sleep disordered breathing but not commonly detected with current technology (and also not easy to treat), some physicians do a full nasopharyngeal endoscopy under propofol anesthesia. I believe it’s incredibly undiagnosed and probably accounts for more than 10% of apnea cases, but most patients are thin instead of obese and so nobody suspects apnea-like disorder. If you needed braces or needed corrective jaw surgery UARS should be ruled out.
ADHD can be somewhat objectively measured with some tests like CPT, but that would also often show abnormalities in sleep disorders.
If your measures of attention, and sleep study, along with wakefulness maintenance test, if warranted, are all normal, and your pulse and blood pressure is normal, and by normal I mean very close to ideal 120/80, with hr below 70 - maybe you are just a mutant.
See a good neuropsych that specializes in sleep disorders. Stanford is one of the better centres for that. They will know what to do if it’s not a sleep issue also.
dx of ADHD and good response do not preclude the possibility of other disorders.
you can obtain treatment that way, and investigate other causes later, you may have to wash out from stimulants for a few weeks for the CPT and WMT, and preferably for sleep studies also, because they increase muscle tone and if your sleep issue is due to some neuromuscular issue, stimulants may partially obscure it. (This is still in research, and most physicians won’t tell you this, as it’s not yet standard of care in most places afaik)