edit: I should add that the link I posted was tweeted by Robb Wolf a few months ago (without commentary), which made me pay more attention to it. I wonder what his current stance on vitamin D is.
Low carb diets have been implicated in high cortisol, e.g:
"[A low-carb] diet will lead to an elevation of catabolic stress hormones, while [a high-carb diet] has been shown to increase thyroid hormone triiodothyronine (T3), increase testosterone, and decrease cortisol, the anti-hair, pro-misery stress hormone (here, here, here (PDF), & here)." (links in original document)
Is this the case? Can it be mitigated by carbing up at the weekend?