I suffered crippling seasonal depression from puberty until the age of 30. That winter I resolved to spend the next year doing everything possible to recover, and if it didn't work I would find some painless way to die.

So I got in awesome shape, I maximized my socialization, ate healthy, spent a lot of time outdoors, etc. I did everything that has been shown as an effective treatment for depression, including a couple of well-planned (and excellent) mushroom trips. My doctor and I had experimented with anti-depressants in my 20s but they had done more harm than good.

Towards the next January I could feel the sense of vague doom chilling my soul that proceeded major depressive episodes. The onset was 2 months later than usual so I decided to weather it. It lasted 3 months - three months of suffering that I believe is unimaginable to a healthy person.

All my efforts had succeeded in cutting my "dark time" by 40%. It was enough that I could live, but had to face the fact that I could never have kids, never take a job of real responsibility, etc. I was barely able to literally survive the winter intact without ending up homeless or worse.

The next year I started taking vitamin D, almost on a whim, because of some vague notion that I didn't have a good dietary source. I can't even express how much it changed my life. My depression utterly disappeared. Completely. My mind cleared. My mental powers (concentration, decision making, etc) doubled, then increased ten-fold. Vitamin D deficiency was my problem all along.

It's taken me a few years to adjust to this new reality. Winter depression was a major part of my worldview, if not actually the defining feature of my life. For a healthy person I think it's like if you didn't get your first pair of shoes until you were in your 30s and your feet were incredibly sensitive. You would suddenly be freed from a slew of cautions and strategies that were no longer needed.

Anyways, long story maybe, but TL;DR Take vitamin D through the winter. You need more if you live further north and/or if you have darker skin