Can ADHD be mostly mental and not physical hyperactivity?

Yes. ADHD does not always show up as visible hyperactivity, especially in adults. Many people experience what clinicians call internal hyperactivity. On the outside they may appear calm, composed, or even low energy, but internally their mind feels constantly active. Thoughts jump rapidly, there is nonstop internal dialogue, mental restlessness, and difficulty turning the brain off. This presentation is especially common in inattentive type ADHD and in adults who learned early to mask outward symptoms. Because it does not match the stereotype of ADHD, it often goes unrecognized for years, even though the internal experience can be exhausting.