That is interesting.

There is a circadian trigger for cluster headache. Spring and fall.
The specialty headache clinics are using melatonin for it, as one treatment.

Also I attended a pain conference (including the cluster headache info last Friday) and they had information about central pain states and receptors.

1) The changing estrogen levels (drops) in female brains can be correlated to triggering migraines (in genetically prone women, not all women) and also may be a factor in fibromyalgia.

2) using opiates for head pain, stimulate glial cells which actually release inflammatory trigger molecules which create MORE pain.

It is possible that Vit D is acting as a hormone in the brain and that day length, light stimulation of the retina where melatonin receptors are, may be involved. Recently studies have found melatonin receptors in the pancreas, and so this circadian action may involve glucose levels as well.

The melatonin information is very interesting of late. It appears that melatonin does much more than facilitate sleep.

Here is a link to the estrogen connection/pain:
http://www.med.umich.edu/opm/newspag.../painbrain.htm

BTW melatonin is synthesized from serotonin, using MethylB12 as a cofactor. So people who do not have adequate B12 will have poor or little melatonin to work with.