After 35 I've more and more noticed how the long darkness affects my mental health. I'm not in northern Norway, so the sun rises all year, but during winter it comes up after I go to work and sets before I go home. So it's basically dark all the time. Supplementing vitamin D seems to help somewhat at least.

I strongly doubt this.

If this was true, seasonal affective disorder would occur in the summer and not the winter, don't you think?

And our ancient ancestors didn't evolve where it is dark. Nor did we evolve where the length of day changes much at all.

The fact that short days and darkness cause so many people problems in the winter tells me that amount of sunlight really matters to us.

And generally speaking, more is better, to an extent.

> The fact that short days and darkness cause so many people problems in the winter tells me that amount of sunlight really matters to us.

For the greatest part of our history we've spent our time outside, not sitting in artificial caves with artificial lights, hiding from the sun. If you want something that tells you why the sun is so important, take this instead.

If that doesn't work for you, remember that having enough exposure to the sun is vital for your body to function properly, both mentally and physically. We'd not be producing "Vitamin D" by being exposed for some time, if the sun wasn't an important factor in our existence.