I'm being evil today and giving everyone homework.
http://psychclassics.yorku.ca/Yerkes/Law/This paper is the work that eventually led to the "inverted U" Yerkes-Dodson law of human performance. The law posits that as stimulation increases, arousal (think alertness not the other, more fun version) increases to a point. After that point further stimulation theoretically results in a decrease in arousal. Sounds valid on the face of it (I love that phrase). There are issues with the initial paper (obviously the first issue would be that the study involved mice, not people).
Now others have done studies trying to apply the conclusions of this paper to humans (and the studies are arguable as to their validity). Your homework is to read this original work and tell me what some of the other issues are with the conclusions.
:D