ADHD stimulants appear to work less by sharpening focus and more by waking up the brain. Brain scans revealed that these medications activate reward and alertness systems, helping children stay interested in tasks they would normally avoid. The drugs even reversed brain patterns linked to sleep deprivation. Researchers say this could complicate ADHD diagnoses if poor sleep is the real underlying problem.

edit here is another article

https://medicine.washu.edu/news/stimulant-adhd-medications-work-differently-than-thought/