Psychologists Yerkes and Dodson demonstrated, in 1908, the relationship between physiological arousal (stress) and performance: Performance increases with arousal, but only to a point. When arousal is too high, performance decreases.
The scientists conducted an experiment with rats running through a maze stimulated by electricity. While gentle electric shocks motivated animals to complete the task, increasing the current caused the opposite effect, where rats performance decreased, and they were only trying to escape. It was determined that only optimum levels of arousal helped to focus attention and motivation on the task at hand, while excessive levels were detrimental.Yerkes RM, Dodson JD (1908). "The relation of strength of stimulus to rapidity of habit-formation". Journal of Comparative Neurology and Psychology.