This article helped greatly in understanding the different levels of stress. Yerkes gave a detailed breakdown on the Inverted-U Hypothesis on the relationship between anxiety and performance. This hypothesis indicated that as arousal increases, then performance also increases and improves but only up to a certain point. If the individuals arousal is pushed beyond the max point then performance will have the inverted affect and diminishes, creating an inverted U shape.

Yerkes and Dodson

1908. The relation of strength of stimulus to rapidity of habit formation. Journal of of Neurological Psychology