Anecdotal: Around the time that I started my day job, I experienced light depressions during winters. For me it was just a winter-dip. I didn't think much of it but my then girlfriend (now wife) recognised that we got into fights much more often than during summers. She studied something along the lines of "Biology, Food and Health", and she thought I'd have a minor Vitamin D deficiency. I've started taking 75mg every day and after about 3 weeks I started seeing more vividly. I remember very well that when I walked out of the office during winter, even though it was near dark, I saw all colors much more bright and it was like a sort of pressure on my sight was gone.

Now, when I'm not in the sun during the day for at least 30-60 minutes (for maintenance in the garden, or working for clients during WFH in the garden), I take on avg 50mg (2000IU) every day and I've never had a dip anymore.