As I have gotten older I've noticed my mood and demeanor does actually change (took me way too long to figure it out) if I don't get enough sun time, even after as little as a week of grey days. So I'll hit the tanning bed (no they are not evil) for 15 minutes a couple of times if I can't get outside (the best is sun but it just isn't always possible). It really does help me, not as good as getting outside in the sun for 30 minutes does but it helps. I am not advocating going to a tanning bed to become a piece of leather, but when you use them responsibly it can help us manage things like SAD, they still aren't a great substitute for Sun but I've used them to help when I really am noticing it.
Also you can check out SAM-E which is a supplement in the U.S., but I think it might be regulated in some of Europe but not sure. I have found SamE is helpful even if I can't get outside or to a tanning bed for a quick session. SamE affects your serotonin levels and helps to balance you out. Obviously if you are taking any other similar meds/supplements probably best to check with a Dr or pharmacists about it, but it can help.
And of course, exercise is a great help and should be one of the first things to try if you don't already do it regularly. IMO doesn't need to be a workout necessarily, but do something active where you will use your muscles and sweat. I say use your muscles because I have tried yoga/stretching before and it doesn't seem to help as much as doing a more physical activity. I know Yoga can be very demanding, but I obviously am not at that level and at least for me I have found doing something that taxes my strength/muscles and forces my body to respond/repair is what seems to do the most good for me.