Probably in days that's too overcast a good serving of fatty fish will help. Vitamin d3 is either synthesized from sun exposure or supplemented through diet, it is a good mood regulator.
Regardless, a good reminder. Fish has also other health benefits. I'd still take some vitamin D supplement on top of that.
As a result of all this care about light I haven't had a SAD winter for most of a decade. I feel "cured" of it enough that spending time in even more northerly places in the winter does not trigger unpleasant depression. That said, us northern peoples should slow down in the winter. Thousands of years of selection has driven this home, and it is nothing to be upset about. We will make up for the slowness of the winter in the long days of summer.