They definitely increase energy levels. A person can stay up for days on them. They are an unwise and partial substitute for sleep but a substitute nonetheless. Like you mention, a large part of what they do is change intrinsic motivation, not intelligence. When you feel "charged" with energy, it is hard to oppose doing intellectual work. Amphetamines are pure energy. And intellect loves pure energy...so does physical work. But its not for free. Amphetamines decrease circulation..they caused Peyronies Disease for me..and I used to take them all the time in college with no issues. Age always brings the true nature of bad habits to bear.