I’m Swiss, but I was born and raised in the Gulf states. When I visit Switzerland during the cold season, I immediately become depressed, irritable, and in a bad mood. I prefer the heat 100%. I don’t think I could live in a place with a cold climate and be happy.
I’d consider vitamin D levels as a possible factor. Deficiency can make you feel that kind of bad and it tends to be an issue around this time of year. Less sun exposure chokes off its production and we’ve had a few months to burn through what gets stashed away in warmer times.
That was my first thought too. My understanding is that Nordic countries do think about vitamin D deficiency (especially this time of year) so it’s common to see stuff like cod liver oil and other ways to supplement vitamin D. Figured Switzerland would be similar.