Hi Maarten,
I am in the same boat as you regarding surgery and the approach, but I am 2 + years post surgery. I suffered with headaches and did find that in the night I would awaken with a headache. I also began to notice my head pain was positional, i.e. depending on which side I would lay on my headache developed on that side, even headache would appear on the back of my head while reclining. I still have headaches, on and off, and use ibuprofen as needed (I am not recommending this though). I also have noticed I have similar symptoms that appear before the pain similar to migraines; i.e. pain in eyes, flashing lights, spots, rings that appear when my eyes are closed sometimes, stomach pain, blurring of vision sometimes, fullness (swelling sensation) to the back of my skull, etc. I am hoping these go away completely over time as they once were daily 24/7 right out of surgery. I hope you find relief sooner than I and perhaps an answer as to why nocturnal only?! I have read migraines can start in the evening/sleeping hours at the same time too. Maybe start a journal of what sensations you notice during the day and maybe you willl begin to see a pattern and can take something before you go to sleep to combat the inflammation so it doesn't wake you in pain?