Hmmm, confusing. I do have an annual seasonal slump when I crash more easily and am overall more sluggish and malaise-ish but the timing is different from the others here. For me it's the winter to spring transition (starting late August/early September, Southern hemisphere).
In the past I interpreted this as a reaction to increasing UV light levels because I have a condition called polymorphic light eruption which involves an immune response (for most people just in the skin but a few, myself included, get a more systemic reaction with flu-like symptoms). Against this theory is the fact that I was always fully sun-protected when leaving the house, and these days I hardly ever leave the house, but still get the 'slump'.
Another suspect was vitamin D but this doesn't really fit either. In the past, whenever I went off supplements for a few months I would get deficient, unsurprising given my sun avoidance. This would indicate that the bulk of my vitamin D comes from supplements and levels should be relatively steady and, indeed, with constant supplementation my levels do test steady.
Now, after reading this thread I'm speculating about changes in melatonin levels as the culprit. I'm pretty certain light is a factor for me in some way. And of course light is a factor in melatonin production. And one of your papers
@JaimeS, I forget which one, said melatonin was lowest during the time of my seasonal slump. Plus, on a day-to-day basis I feel mentally better on bright days, the sun lifts my mood, but physically I'm actually better on overcast days (effect is independent of temperature) with the weird result that in bright sunshine I feel lousy but uplifted.
I'll finish the post the way I started it: Hmmm, confusing.
