Chronic severe nocturnal hypnic headaches

My hypnic headaches have completely stopped. I have not had a headache in over four months. After relevant tests and scans (which showed minor, age-appropriate white spots) came back normal, my neurologist diagnosed the headaches I experienced for over two years—which occurred specifically upon waking from sleep—as "hypnic headaches."
While my neurologist had no definitive explanation for why this works, I began taking one 200mg caffeine pill before bed, and the headaches ceased immediately—literally from the first night. As a lifetime black tea and occasional coffee consumer, I changed nothing else in my diet, exercise, or supplement routine.
I would like to understand the mechanism of this, specifically if it is as simple as caffeine constricting blood vessels that were previously expanding/dilating, or if it is related to hypothalamic activity. I hope sharing that a simple, OTC caffeine pill solved my chronic issue helps others.