Hi,

In my personal experience, as well as my academic research, I've found that stimulants rev up the frontal cortical region of the brain (specifically the dopaminergic system and the adrenal system) which compensates for the naturally low level of activity of these systems in the frontal cortex.

However, while the drugs/caffeine's effect on the frontal region brings activity levels up to a normal, functioning level (producing an ability to concentrate, reducing impulsiveness, and lowering hyperactivity) it is a stimulant --- and acts on other areas of the central nervous system in a stimulating fashion. Which is why one gets the effects of faster heart rate, inability to sleep, loss/diminished appetite, and a mild increase in blood pressure. The calming effect that some experience while on stimulants is due to the focus it produces, which lowers impulsiveness, and is especially noticeable in people with the hyperactive type or combined types of ADHD.

Also, caffeine increases concentration and brain processing speed in all people, not just ADHDers; as do psychotropic stimulants (e.g. Adderall) which has a calming/focusing effect on non-ADHDers. Which is why it's the drug of choice on college campuses and even on Wall street -- if it really made people without ADHD bounce around physically and mentally, I doubt this growing phenomenon would exist. However, people without ADHD only require low doses to achieve the same effect, as their brains have a higher/normal level of frontal cortical activity. There's a great article about it in an issue of National Geographic. Sugar, however, usually exacerbates ADHD symptoms, since it's processed quickly by the body, causing a spike in blood sugar levels.

"Stimulants don't just work this way on people diagnosed with ADHD: most people experience this same effect, regardless. In the words of Michael Rutter, specialist in child disorders at the University of London, "Stimulants… tend to improve attention and reduce activity in all people, children and adults, irrespective or whether or not they are hyperactive." The only difference with a hyperactive person is that the effect of the drug is more noticeable."-- http://www.santa.inuk.com/FAQs.htm#how

Ok, sorry for going on forever....I hope someone finds this useful.