The optimal level refers to the balance point where an individual can achieve peak performance without being under-stimulated or over-stressed. The optimal level of arousal changes based on the task. For example, a lower arousal level is ideal for concentration-heavy tasks (studying, solving complex problems), while higher arousal may be necessary for physical tasks (sports, public speaking). Tools like heart rate monitors or galvanic skin response devices can quantify physiological arousal, while psychological scales measure subjective arousal (e.g., feelings of stress, excitement).