A feeling (and now a word) I am very familiar with:
busy brain
n. A mental state that includes racing thoughts, anxiety, lack of focus, and sleeplessness.
Earliest citation:
Medication is swelling his feet and keeping him awake at night, while his mind is under siege from “busy brain”, a side effect of the medication.
—Peter Ellingsen, “Suddenly, there’s nowhere to hide,” The Age, December 10, 2001.