ADD vs. Normal Response to Stimulation

Everyone has a range of stimulation within which they function. They are excited at the higher end and relaxed at the lower end. But everyone also has a level of stimulation below which they are bored and above which they are overstressed.
ADD/ADHD people tend to have a problem at both ends. They need higher stimulation than normal people, and are bored with routine levels. This causes an inability to concentrate. But they are also overloaded or stressed too easily. It is hard for them to focus on one thing at a time when there are multiple stimuli.
Stimulants help both problems. They turn on the brain to enable concentration on the lower level stimuli. Stimulants also allow focus on one thing at a time when there are multiple stimuli, which helps the ADD person to be less hyper and more relaxed.