My bought with these horrible nocturnal hypnic headaches came on 2 years ago in the early fall. I was desperate to find the answers you seek and any relief I could find. Unfortunately, I never could confirm the source, but after trying many of the drugs, caffeine, melatonin, etc., my neurologist gave me a steroid step down prescription late December and by mid January they stopped. I only had 1 or 2 milder episodes in the following month and nothing after. My neurologist hypothesized that a nerve had been irritated and that the steroid calmed it back down. Hope this helps you. Truly the most frightening and painful thing I’d experienced (outside of childbirth!). Good luck!