Very interesting, yes. I go from severe to almost moderate most summers, the shift usually starts around May and peaks in September. I don't get outside at all in the winter, not that there's much light to be had anyway, so I supplement in the winter months. Hopefully this year I will get out more in the summer, now that we're on the ground floor and with a garden. I have used a solarium a few times, but usually in the spring when it might not be as helpful.
In the fall, before I start to get more severe again, I will try to go a few times a week to see if I can stave off the months of being stuck in bed. I do feel a lot better from using my IR sauna too, but that's more because of better oxygen supply to sore muscles and better skin. When I'm in a severe phase I can't manage that either, anyway. Seeing a chiropractor regularly helps a lot with headaches and knotted muscles, but again impossible at my worst. So if using a solarium bed can help me extend those interventions as well, that would make my winter months ten times better.