I get double - Triple depressed due to the dark, dull, dreary 360 odd days per year in the UK!!!
I'm now living in the Caribbean and it has saved my life - that's for sure! I'd have long comitted suicide otherwise. Yes it's all a lack of vitamin D so Im told which is what we naturally absorb from the sun (if & when it manages to poke through the typical thick cloud cover).
But I believe it's more than just that - I wake up in the UK and already befrore I even look outside and see the weather am in a prdefined low & dark mood - so explain that?!
Anyway, if I had to live there again I'd have to buy a huge UV bulb and fill a room with sand to make an escape from certain doom.