Arousal is set by default by whatever is on the screen: a fast open drives it up, a dead afternoon lets it fall. Neither default is likely to match the level your decisions need, which is the practical use of the yerkes-dodson-law.
Bringing it down: slow breathing, a longer exhale, standing up, reducing size, closing extra charts, a short walk away from the desk. Bringing it up: a break with movement, changing posture, narrowing the task to one instrument, setting a specific thing to watch for.
Measure it crudely and often. A one-to-ten rating in the journal before and after each trade takes seconds, and after a month it will show you the two or three levels at which you make most of your mistakes.
Related: yerkes-dodson-law, stress-response, box-breathing, behaviour-journal