IME there is an additional mental fog during the fall and winter seasons. I think there are a handful of reasons that are mostly a byproduct of shorter days and colder temps. More barriers to going outside when it’s cold (more layers, jackets), less social gatherings during the season, it being dark so early limits outdoor activities. Being able to do more helps my mental health for sure, which is much easier in the summer months.
Edit after seeing other comments: I was able to reach a “toxic” level of vitamin D last time I tried it, daily headaches, even just following the recommended amount. It took me a while to realize what was going on. I might have to try again with a smaller dose though.