@Corngold This is one of the main "stress points" of my own system. Probably related to poor posture from myopia and heavy computer use for work and studying. I have found that in caffeine withdrawal, my headaches are localized to exactly around the atlas vertebra (perhaps somewhat different from your case). I can even improve headache pain symptoms somewhat by carefully positioning my head to improve blood flow (you get a feel for it...). IME it is some kind of bloodflow/nitric oxide/serotonin related thing. Smooth muscle. It is one of those things where improvement in the pain is expressed by several minutes in an improved position, which is probably why that is not very much talked about. You need to be quiet and conscious and patient.
Used to be around once a month and triggered by miscellaneous physiological stresses (one time from grounding for several hours, another from being in a very high EMF environment (like, validated by measuring E-Field, not fantasizing here). Nasty headaches radiating from the atlas vertebra. Almost always I would vomit after about an hour of pain, and then the endorphins from the vomiting would mostly take the pain away. Perhaps vagal nerve compression/activation was causing that vomiting? As in the Nystagmus mechanism of action for motion sickness (which I also have, go figure). I suppose this technically makes it a migraine but I lack many of the other migraine symptoms. Migraine has a well-established connection to serotonin.
Peating alone (mostly means increasing sugar for me) has eliminated that. Has not happened in that way again even once. Still happens in that same way with caffeine withdrawal... probably not a good sign. I have started to carry caffeine pills for when I am on the road to prevent withdrawal, which I get after even one day without my two pots of radioactive waste.
Addendum. Vertebral arteries are looped through the vertebrae. Which probably makes them very sensitive to variation in dilation caused by serotonin/nitric oxide/caffeine.
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Protip: 5-hydroxytryptamine, or 5-HT, is a name for serotonin sometimes used when Science wants to discuss its negative consequences without cutting into SSRI sales.